avcodec/mpegvideo: Fix undefined left shift of negative numbers

Fixes the rv20-1239 FATE-test.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mpegvideo.c

@@ -843,7 +843,7 @@ static inline int hpel_motion_lowres(MpegEncContext *s,
         s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, src,
                                  s->linesize, s->linesize,
                                  w + 1, (h + 1) << field_based,
-                                 src_x, src_y   << field_based,
+                                 src_x, src_y * (1 << field_based),
                                  h_edge_pos, v_edge_pos);
         src = s->sc.edge_emu_buffer;
         emu = 1;
@@ -945,7 +945,7 @@ static av_always_inline void mpeg_motion_lowres(MpegEncContext *s,
         s->vdsp.emulated_edge_mc(s->sc.edge_emu_buffer, ptr_y,
                                  linesize >> field_based, linesize >> field_based,
                                  17, 17 + field_based,
-                                src_x, src_y << field_based, h_edge_pos,
+                                src_x, src_y * (1 << field_based), h_edge_pos,
                                 v_edge_pos);
         ptr_y = s->sc.edge_emu_buffer;
         if (!CONFIG_GRAY || !(s->avctx->flags & AV_CODEC_FLAG_GRAY)) {
@@ -956,12 +956,12 @@ static av_always_inline void mpeg_motion_lowres(MpegEncContext *s,
             s->vdsp.emulated_edge_mc(ubuf,  ptr_cb,
                                      uvlinesize >> field_based, uvlinesize >> field_based,
                                      9, 9 + field_based,
-                                    uvsrc_x, uvsrc_y << field_based,
+                                    uvsrc_x, uvsrc_y * (1 << field_based),
                                     h_edge_pos >> 1, v_edge_pos >> 1);
             s->vdsp.emulated_edge_mc(vbuf,  ptr_cr,
                                      uvlinesize >> field_based,uvlinesize >> field_based,
                                      9, 9 + field_based,
-                                    uvsrc_x, uvsrc_y << field_based,
+                                    uvsrc_x, uvsrc_y * (1 << field_based),
                                     h_edge_pos >> 1, v_edge_pos >> 1);
             ptr_cb = ubuf;
             ptr_cr = vbuf;