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

[balazske]

Summary

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

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

 //===--- SuspiciousReallocUsageCheck.cpp - clang-tidy----------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
public:
   }
   bool VisitBinaryOperator(const BinaryOperator *S) {
     if (S->getOpcode() == BO_Assign)
       return isAccessForVar(S->getRHS());
     return false;
   }
   bool VisitStmt(const Stmt *S) {
     for (const Stmt *Child : S->children())
-      if (Visit(Child))
+      if (Child && Visit(Child))
         return true;
     return false;
   }
 };
 
 } // namespace
 
 namespace clang {

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

 // RUN: %check_clang_tidy %s bugprone-suspicious-realloc-usage %t
 
 void *realloc(void *, __SIZE_TYPE__);
 
 namespace std {
   using ::realloc;
 }
 
void no_warn_if_copy_exists_before3() {
   g_p = realloc(g_p, 111);
 }
 
 void warn_if_copy_exists_after(void *p) {
   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]
   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]
+}