smalloc: fix bad assert for zero length data

If the data length passed to smalloc.alloc() the array_length will be
zero, causing an overflow check to fail. This prevents that from
happening.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>

src/smalloc.cc

@@ -132,7 +132,7 @@ void CallbackInfo::WeakCallback(Isolate* isolate, Local<Object> object) {
       object->GetIndexedPropertiesExternalArrayDataType();
   size_t array_size = ExternalArraySize(array_type);
   CHECK_GT(array_size, 0);
-  if (array_size > 1) {
+  if (array_size > 1 && array_data != NULL) {
     CHECK_GT(array_length * array_size, array_length);  // Overflow check.
     array_length *= array_size;
   }