[clang-tidy] Fix crash in bugprone-suspicious-realloc-usage.

The problem occurs if a statement is found by the checker that has a null child.
Fixes issue #59518.

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D140194

clang-tools-extra/clang-tidy/bugprone/SuspiciousReallocUsageCheck.cpp

@@ -95,7 +95,7 @@ class FindAssignToVarBefore
   }
   bool VisitStmt(const Stmt *S) {
     for (const Stmt *Child : S->children())
-      if (Visit(Child))
+      if (Child && Visit(Child))
         return true;
     return false;
   }

clang-tools-extra/test/clang-tidy/checkers/bugprone/suspicious-realloc-usage.cpp

@@ -100,3 +100,10 @@ void warn_if_copy_exists_after(void *p) {
   // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: 'p' may be set to null if 'realloc' fails, which may result in a leak of the original buffer [bugprone-suspicious-realloc-usage]
   void *q = p;
 }
+
+void test_null_child(void *p) {
+  for (;;)
+    break;
+  p = realloc(p, 111);
+  // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: 'p' may be set to null if 'realloc' fails, which may result in a leak of the original buffer [bugprone-suspicious-realloc-usage]
+}