Bug 877087 - Clear mDsFile after the OPP connection is dropped, r=gyeh

dom/bluetooth/BluetoothOppManager.cpp

@@ -461,6 +461,7 @@ BluetoothOppManager::AfterOppDisconnected()
   mConnected = false;
   mLastCommand = 0;
   mPacketLeftLength = 0;
+  mDsFile = nullptr;
 
   ClearQueue();
 
@@ -494,6 +495,7 @@ BluetoothOppManager::DeleteReceivedFile()
 
   if (mDsFile && mDsFile->mFile) {
     mDsFile->mFile->Remove(false);
+    mDsFile = nullptr;
   }
 }