avcodec/mpeg4videodec: Fix 2 integer overflows in get_amv()

Fixes: signed integer overflow: -144876608 * 16 cannot be represented in type 'int'
Fixes: 22782/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-6039584977977344

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

libavcodec/mpeg4videodec.c

@@ -610,7 +610,7 @@ static inline int get_amv(Mpeg4DecContext *ctx, int n)
             dy -= 1 << (shift + a + 1);
         else
             dx -= 1 << (shift + a + 1);
-        mb_v = s->sprite_offset[0][n] + dx * s->mb_x * 16 + dy * s->mb_y * 16;
+        mb_v = s->sprite_offset[0][n] + dx * s->mb_x * 16U + dy * s->mb_y * 16U;
 
         sum = 0;
         for (y = 0; y < 16; y++) {