demux: avi: fix invalid deref

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>
(cherry picked from commit 16504feba9aa9ea946dc711eed113943f5285f05)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

modules/demux/avi/avi.c

@@ -3003,7 +3003,8 @@ static void AVI_ExtractSubtitle( demux_t *p_demux,
             p_indx->i_entriesinuse > 0 )
         {
             if( vlc_stream_Seek( p_demux->s, p_indx->idx.super[0].i_offset ) ||
-                AVI_ChunkRead( p_demux->s, &ck, NULL  ) )
+                AVI_ChunkRead( p_demux->s, &ck, NULL  ) ||
+                ck.common.i_chunk_fourcc != AVIFOURCC_indx )
                 goto exit;
             p_indx = &ck.indx;
         }