Entries are always at least 8 bytes per the parsing code, so if we
see an impossible entry count avoid massive allocations. This is
similar to an existing check in mov_read_stsc().

Since ff_mov_read_stsd_entries() does eof checks, an alternative
approach could be to clamp the entry count to atom.size / 8.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>