Mark all 12 block unused when there are no coded blocks

Fixes 422/444 artifacts

Originally committed as revision 4708 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg12.c

@@ -1436,7 +1436,7 @@ static int mpeg_decode_mb(MpegEncContext *s,
                 }
             }
         }else{
-            for(i=0;i<6;i++)
+            for(i=0;i<12;i++)
                 s->block_last_index[i] = -1;
         }
     }