Fix null pointer exception in FieldMatchers by accounting for primitive class literals

[sebastiantoh]

Fixes: #5589

Before

I have an Error Prone checker that implements MemberSelectTreeMatcher:

public final class MyBugChecker extends BugChecker implements MemberSelectTreeMatcher {

    private static final Matcher<ExpressionTree> MATCHER =
            FieldMatchers.anyFieldInClass("com.example.SomeClass");

    @Override
    public Description matchMemberSelect(MemberSelectTree tree, VisitorState state) {
        if (MATCHER.matches(tree, state)) {
            ...
        }
        ...
    }
}

When this checker runs on a class that references a primitive class literal, e.g.:

class Test {
    Class<?> clazz = long.class;
}

a NullPointerException is thrown. See the truncated stack trace below:

java.lang.AssertionError: An unhandled exception was thrown by the Error Prone static analysis plugin.
     Please report this at https://github.com/google/error-prone/issues/new and include the following, as well as a reproducing code sample (if possible):
  
     error-prone version: unknown version
     BugPattern: MyBugChecker
     Stack Trace:
     java.lang.NullPointerException: Cannot invoke "com.sun.tools.javac.code.Symbol$ClassSymbol.getQualifiedName()" because "classSymbol" is null
  	at com.google.errorprone.matchers.FieldMatchers$1.classIsAppropriate(FieldMatchers.java:42)
  	at com.google.errorprone.matchers.FieldMatchers$FieldReferenceMatcher.isSymbolFieldInAppropriateClass(FieldMatchers.java:95)
  	at com.google.errorprone.matchers.FieldMatchers$FieldReferenceMatcher.matches(FieldMatchers.java:83)
  	at com.google.errorprone.matchers.FieldMatchers$FieldReferenceMatcher.matches(FieldMatchers.java:80)
  	at com.example.errorprone.MyBugChecker.matchMemberSelect(MyBugChecker.java)
  	at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:509)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitMemberSelect(ErrorProneScanner.java:804)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitMemberSelect(ErrorProneScanner.java:170)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:2584)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:92)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:95)
  	at jdk.compiler/com.sun.source.util.TreeScanner.visitVariable(TreeScanner.java:242)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitVariable(ErrorProneScanner.java:1017)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitVariable(ErrorProneScanner.java:170)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:1084)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:92)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:95)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scan(TreeScanner.java:110)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:118)
  	at jdk.compiler/com.sun.source.util.TreeScanner.visitClass(TreeScanner.java:202)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:619)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:170)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:899)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:92)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scan(TreeScanner.java:110)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:118)
  	at jdk.compiler/com.sun.source.util.TreeScanner.visitCompilationUnit(TreeScanner.java:151)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:631)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:170)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:627)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:66)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:58)
  	at com.google.errorprone.scanner.ErrorProneScannerTransformer.apply(ErrorProneScannerTransformer.java:43)
  	at com.google.errorprone.ErrorProneAnalyzer.finished(ErrorProneAnalyzer.java:231)
  	at jdk.compiler/com.sun.tools.javac.api.MultiTaskListener.finished(MultiTaskListener.java:133)
  	at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1423)
  	at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1370)
  	at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:955)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.lambda$doCall$0(JavacTaskImpl.java:104)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.invocationHelper(JavacTaskImpl.java:152)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:100)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:94)
        ...

After

Fix null pointer exception

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