demux: mp4: fix heap buffer overflow (fix #12266)

(cherry picked from commit 26258915a956bd1150d4d369d476770af21c71b7)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mp4/mp4.c

@@ -1747,6 +1747,8 @@ static int TrackCreateChunksIndex( demux_t *p_demux,
         i_last = BOXDATA(p_stsc)->i_first_chunk[i_index] - 1;
     }
 
+    if ( p_demux_track->i_chunk_count )
+    {
         p_demux_track->chunk[0].i_sample_first = 0;
         for( i_chunk = 1; i_chunk < p_demux_track->i_chunk_count; i_chunk++ )
         {
@@ -1754,6 +1756,7 @@ static int TrackCreateChunksIndex( demux_t *p_demux,
                 p_demux_track->chunk[i_chunk-1].i_sample_first +
                     p_demux_track->chunk[i_chunk-1].i_sample_count;
         }
+    }
 
     msg_Dbg( p_demux, "track[Id 0x%x] read %d chunk",
              p_demux_track->i_track_ID, p_demux_track->i_chunk_count );