Bug 1589953 - Initialize `read` in AlternativeDataStreamListener::OnDataAvailable(); r=edgar

Differential Revision: https://phabricator.services.mozilla.com/D49849

--HG--
extra : moz-landing-system : lando

dom/fetch/FetchDriver.cpp

@@ -261,7 +261,7 @@ AlternativeDataStreamListener::OnDataAvailable(nsIRequest* aRequest,
                                                uint32_t aCount) {
   if (mStatus == AlternativeDataStreamListener::LOADING) {
     MOZ_ASSERT(mPipeAlternativeOutputStream);
-    uint32_t read;
+    uint32_t read = 0;
     return aInputStream->ReadSegments(
         NS_CopySegmentToStream, mPipeAlternativeOutputStream, aCount, &read);
   }