Fix leak of frame_duration_buffer in mov_fix_index().

Should be unconditionally freed at the end of mov_fix_index() in
case it hasn't been used during the fix up.

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Sasi Inguva <isasi-at-google.com@ffmpeg.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
