[gn] Enable stricter build flags

src/api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 9387 matching lines @@
                              PersistentValueVector<debug::Script>& scripts) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
   ENTER_V8(isolate);
   // TODO(kozyatinskiy): remove this GC once tests are dealt with.
   isolate->heap()->CollectAllGarbage(i::Heap::kFinalizeIncrementalMarkingMask,
                                      i::GarbageCollectionReason::kDebugger);
   {
     i::DisallowHeapAllocation no_gc;
     i::Script::Iterator iterator(isolate);
     i::Script* script;
-    while ((script = iterator.Next())) {
+    while ((script = iterator.Next()) != nullptr) {
       if (!script->IsUserJavaScript()) continue;
       if (script->HasValidSource()) {
         i::HandleScope handle_scope(isolate);
         i::Handle<i::Script> script_handle(script, isolate);
         scripts.Append(ToApiHandle<Script>(script_handle));
       }
     }
   }
 }
 
@@ skipping 834 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8