Use the correct test

Refs #791

modules/codec/libmpeg2.c

@@ -294,9 +294,7 @@ static picture_t *DecodeBlock( decoder_t *p_dec, block_t **pp_block )
             p_block->i_buffer = 0;
             break;
 
-#ifdef STATE_SEQUENCE_MODIFIED
-/* FIXME - the above ifdef is always FALSE, even with libmpeg2-0.5.0
- * Need to improve the implementation to avoid invalid/unref pictures */
+#if MPEG2_RELEASE >= MPEG2_VERSION (0, 5, 0)
 
         case STATE_SEQUENCE_MODIFIED:
             GetAR( p_dec );