Try to honor even completely invalid motion vectors as far as is

easy.
Fixes issue961.

Originally committed as revision 18344 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo_common.h

@@ -315,6 +315,7 @@ if(s->quarter_sample)
                s->codec_id == CODEC_ID_MPEG1VIDEO){
                 av_log(s->avctx,AV_LOG_DEBUG,
                         "MPEG motion vector out of boundary\n");
+                if(!s->chroma_y_shift)
                 return ;
             }
             ff_emulated_edge_mc(s->edge_emu_buffer, ptr_y, s->linesize,