lavf/mov: don't read outside frag_index bounds

Potentially fixes:
https://bugs.chromium.org/p/chromium/issues/detail?id=786269#c1

In theory, the crash can be triggered by an invalid stream that has
either tfdt or trun outside of the moof

Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
