avoid clearing block[] if its not used at all

Originally committed as revision 4148 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg12.c

@@ -1140,6 +1140,12 @@ static int mpeg_decode_mb(MpegEncContext *s,
     dprintf("mb_type=%x\n", mb_type);
 //    motion_type = 0; /* avoid warning */
     if (IS_INTRA(mb_type)) {
+        s->dsp.clear_blocks(s->block[0]);
+    
+        if(!s->chroma_y_shift){
+            s->dsp.clear_blocks(s->block[6]);
+        }
+    
         /* compute dct type */
         if (s->picture_structure == PICT_FRAME && //FIXME add a interlaced_dct coded var?
             !s->frame_pred_frame_dct) {
@@ -1357,8 +1363,13 @@ static int mpeg_decode_mb(MpegEncContext *s,
         }
         
         s->mb_intra = 0;
-
         if (HAS_CBP(mb_type)) {
+            s->dsp.clear_blocks(s->block[0]);
+        
+            if(!s->chroma_y_shift){
+                s->dsp.clear_blocks(s->block[6]);
+            }
+
             cbp = get_vlc2(&s->gb, mb_pat_vlc.table, MB_PAT_VLC_BITS, 1);
             if (cbp < 0 || ((cbp == 0) && (s->chroma_format < 2)) ){
                 av_log(s->avctx, AV_LOG_ERROR, "invalid cbp at %d %d\n", s->mb_x, s->mb_y);
@@ -2578,10 +2589,6 @@ static int mpeg_decode_slice(Mpeg1Context *s1, int mb_y,
             XVMC_init_block(s);//set s->block
 #endif
 
-	s->dsp.clear_blocks(s->block[0]);
-        if(!s->chroma_y_shift){
-            s->dsp.clear_blocks(s->block[6]);
-        }
         ret = mpeg_decode_mb(s, s->block);
         s->chroma_qscale= s->qscale;