gh-146093: Fix csv _set_str(): check if PyUnicode_DecodeASCII() failed

The function can fail on a memory allocation failure.

Bug reported by devdanzin.

Author: vstinner

Modules/_csv.c

@@ -315,8 +315,12 @@ _set_char(const char *name, Py_UCS4 *target, PyObject *src, Py_UCS4 dflt)
 static int
 _set_str(const char *name, PyObject **target, PyObject *src, const char *dflt)
 {
-    if (src == NULL)
+    if (src == NULL) {
         *target = PyUnicode_DecodeASCII(dflt, strlen(dflt), NULL);
+        if (*target == NULL) {
+            return -1;
+        }
+    }
     else {
         if (!PyUnicode_Check(src)) {
             PyErr_Format(PyExc_TypeError,