[ServiceWorker] Add NULL check of ExecutionContext in FetchBodyStream::readAsync

Source/modules/serviceworkers/FetchBodyStream.cpp

Index: Source/modules/serviceworkers/FetchBodyStream.cpp
diff --git a/Source/modules/serviceworkers/FetchBodyStream.cpp b/Source/modules/serviceworkers/FetchBodyStream.cpp
index 386b642194a5c313aa2cd89e28e9fa31d0a580c6..cd8a1a59ed2eb214c8187f2f8f3244b407e15382 100644
--- a/Source/modules/serviceworkers/FetchBodyStream.cpp
+++ b/Source/modules/serviceworkers/FetchBodyStream.cpp
@@ -66,8 +66,15 @@ ScriptPromise FetchBodyStream::readAsync(ScriptState* scriptState, ResponseType
         ASSERT_NOT_REACHED();
     }
 
+    ExecutionContext* executionContext = scriptState->executionContext();
+    if (!executionContext) {
+        m_resolver->reject(V8ThrowException::createTypeError("ExecutionContext terminated", scriptState->isolate()));
+        m_resolver.clear();
+        return promise;
+    }
+
     m_loader = adoptPtr(new FileReaderLoader(readType, this));
-    m_loader->start(scriptState->executionContext(), m_blobDataHandle);
+    m_loader->start(executionContext, m_blobDataHandle);
 
     return promise;
 }