Bug 1270689 - Clear WidevineDecryptor::mCallback in WidevineDecryptor::DecryptingComplete(). r=gerald

The OnSessionClosed() callback is happening on a timer, after
WidevineDecryptor::DecryptingComplete() has been called. So mCallback could
actually be non-null because DecryptingComplete() has been called, but the
object pointed to by mCallback has been deallocated, and mCallback is a
dangling pointer.

MozReview-Commit-ID: 4xdHYRn7EAS

--HG--
extra : source : 55d79997aa3e3e2133d9155c37f4435317b44061

dom/media/gmp/widevine-adapter/WidevineDecryptor.cpp

@@ -187,6 +187,7 @@ WidevineDecryptor::DecryptingComplete()
 {
   Log("WidevineDecryptor::DecryptingComplete() this=%p", this);
   mCDM = nullptr;
+  mCallback = nullptr;
   Release();
 }