avcodec/mpeg12dec: Fix runtime error: left shift of negative value

Fixes: 608/clusterfuzz-testcase-603978286392934

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg12dec.c

@@ -865,8 +865,8 @@ static int mpeg_decode_mb(MpegEncContext *s, int16_t block[12][64])
                                                    s->last_mv[i][0][1]);
                             /* full_pel: only for MPEG-1 */
                             if (s->full_pel[i]) {
-                                s->mv[i][0][0] <<= 1;
-                                s->mv[i][0][1] <<= 1;
+                                s->mv[i][0][0] *= 2;
+                                s->mv[i][0][1] *= 2;
                             }
                         }
                     }