Bug 1330273 - Add some security checks about the use of the string buffer in FileReader, r=bkelly

dom/file/FileReader.cpp

@@ -333,6 +333,7 @@ FileReader::DoReadData(uint64_t aCount)
     }
 
     uint32_t bytesRead = 0;
+    MOZ_DIAGNOSTIC_ASSERT(mFileData);
     mAsyncStream->Read(mFileData + mDataLen, aCount, &bytesRead);
     MOZ_ASSERT(bytesRead == aCount, "failed to read data");
   }
@@ -731,14 +732,16 @@ FileReader::Notify(Status aStatus)
 void
 FileReader::Shutdown()
 {
-  FreeFileData();
-  mResultArrayBuffer = nullptr;
+  mReadyState = DONE;
 
   if (mAsyncStream) {
     mAsyncStream->Close();
     mAsyncStream = nullptr;
   }
 
+  FreeFileData();
+  mResultArrayBuffer = nullptr;
+
   if (mWorkerPrivate && mBusyCount != 0) {
     ReleaseWorker();
     mWorkerPrivate = nullptr;