Bug 787805 - Null check mInput in FileMediaResource::EnsureLengthInitialized(). r=roc

content/media/MediaResource.cpp

@@ -1031,7 +1031,7 @@ private:
 void FileMediaResource::EnsureLengthInitialized()
 {
   mLock.AssertCurrentThreadOwns();
-  if (mSizeInitialized) {
+  if (mSizeInitialized || !mInput) {
     return;
   }
   mSizeInitialized = true;