When reading frames, we need to use the fragment for the correct
stream.  Sometimes the "current" fragment is not the same as the one
the frame is for.

Found by Chromium's ClusterFuzz:
https://crbug.com/906392 and https://crbug.com/915524Signed-off-by: Jacob Trimble <modmaker@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>