gh-146093: Fix csv _set_str(): check if PyUnicode_DecodeASCII() failed (GH-146113)

The function can fail on a memory allocation failure.

Bug reported by devdanzin.
(cherry picked from commit 724c7c8)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

Modules/_csv.c

@@ -302,8 +302,12 @@ _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 (src == Py_None)
             *target = NULL;