mov: fix heap buffer overflow

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -1830,7 +1830,7 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
 
         current_dts -= sc->dts_shift;
 
-        if (!sc->sample_count)
+        if (!sc->sample_count || st->nb_index_entries)
             return;
         if (sc->sample_count >= UINT_MAX / sizeof(*st->index_entries))
             return;