Bug 1261900: [MSE] P2. Prevent assertion if first media segment contains no usable frames. r=gerald

Encountered while using YouTube MSE/webm compliance test.

dom/media/mediasource/TrackBuffersManager.cpp

@@ -1642,10 +1642,12 @@ TrackBuffersManager::InsertFrames(TrackBuffer& aSamples,
   // We allow a fuzz factor in our interval of half a frame length,
   // as fuzz is +/- value, giving an effective leeway of a full frame
   // length.
+  if (aIntervals.Length()) {
     TimeIntervals range(aIntervals);
     range.SetFuzz(trackBuffer.mLongestFrameDuration / 2);
     trackBuffer.mSanitizedBufferedRanges += range;
   }
+}
 
 void
 TrackBuffersManager::RemoveFrames(const TimeIntervals& aIntervals,