Do checks when receiving external string resources through the API.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index f3f5b16914ea2c5f4a146575f8dc3f06a587b58b..f152175e0b5ed0c15a2b82eed0fb3311c484be16 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4790,6 +4790,7 @@ Local<String> v8::String::NewExternal(
   EnsureInitializedForIsolate(isolate, "v8::String::NewExternal()");
   LOG_API(isolate, "String::NewExternal");
   ENTER_V8(isolate);
+  CHECK(resource && resource->data());
   i::Handle<i::String> result = NewExternalStringHandle(isolate, resource);
   isolate->heap()->external_string_table()->AddString(*result);
   return Utils::ToLocal(result);
@@ -4810,6 +4811,7 @@ bool v8::String::MakeExternal(v8::String::ExternalStringResource* resource) {
   if (isolate->heap()->IsInGCPostProcessing()) {
     return false;
   }
+  CHECK(resource && resource->data());
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     isolate->heap()->external_string_table()->AddString(*obj);
@@ -4824,6 +4826,7 @@ Local<String> v8::String::NewExternal(
   EnsureInitializedForIsolate(isolate, "v8::String::NewExternal()");
   LOG_API(isolate, "String::NewExternal");
   ENTER_V8(isolate);
+  CHECK(resource && resource->data());
   i::Handle<i::String> result = NewExternalAsciiStringHandle(isolate, resource);
   isolate->heap()->external_string_table()->AddString(*result);
   return Utils::ToLocal(result);
@@ -4845,6 +4848,7 @@ bool v8::String::MakeExternal(
   if (isolate->heap()->IsInGCPostProcessing()) {
     return false;
   }
+  CHECK(resource && resource->data());
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     isolate->heap()->external_string_table()->AddString(*obj);