avcodec/mpeg4video: Fix runtime error: left shift of negative value

Fixes: 644/clusterfuzz-testcase-4726434209726464
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg4videodec.c

@@ -510,7 +510,7 @@ static inline int get_amv(Mpeg4DecContext *ctx, int n)
         if (ctx->divx_version == 500 && ctx->divx_build == 413)
             sum = s->sprite_offset[0][n] / (1 << (a - s->quarter_sample));
         else
-            sum = RSHIFT(s->sprite_offset[0][n] << s->quarter_sample, a);
+            sum = RSHIFT(s->sprite_offset[0][n] * (1 << s->quarter_sample), a);
     } else {
         dx    = s->sprite_delta[n][0];
         dy    = s->sprite_delta[n][1];

libavcodec/mpegvideo_motion.c

@@ -48,8 +48,8 @@ static void gmc1_motion(MpegEncContext *s,
     motion_y   = s->sprite_offset[0][1];
     src_x      = s->mb_x * 16 + (motion_x >> (s->sprite_warping_accuracy + 1));
     src_y      = s->mb_y * 16 + (motion_y >> (s->sprite_warping_accuracy + 1));
-    motion_x <<= (3 - s->sprite_warping_accuracy);
-    motion_y <<= (3 - s->sprite_warping_accuracy);
+    motion_x *= 1 << (3 - s->sprite_warping_accuracy);
+    motion_y *= 1 << (3 - s->sprite_warping_accuracy);
     src_x      = av_clip(src_x, -16, s->width);
     if (src_x == s->width)
         motion_x = 0;
@@ -95,8 +95,8 @@ static void gmc1_motion(MpegEncContext *s,
     motion_y   = s->sprite_offset[1][1];
     src_x      = s->mb_x * 8 + (motion_x >> (s->sprite_warping_accuracy + 1));
     src_y      = s->mb_y * 8 + (motion_y >> (s->sprite_warping_accuracy + 1));
-    motion_x <<= (3 - s->sprite_warping_accuracy);
-    motion_y <<= (3 - s->sprite_warping_accuracy);
+    motion_x  *= 1 << (3 - s->sprite_warping_accuracy);
+    motion_y  *= 1 << (3 - s->sprite_warping_accuracy);
     src_x      = av_clip(src_x, -8, s->width >> 1);
     if (src_x == s->width >> 1)
         motion_x = 0;