Revert r18344. Fixes at least issue 1218, 1221, and 1187), see master

issue 1221 for details.
Briefly, out of bounds field mvs in field pictures would cause a crash.

Originally committed as revision 19575 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo_common.h

@@ -315,7 +315,6 @@ 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,