avcodec/mpeg4videodec: more unsigned in amv computation

Fixes: signed integer overflow: -2147483648 + -1048576 cannot be represented in type 'int'
Fixes: 59365/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-642654923954585

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

@@ -861,7 +861,7 @@ static inline int get_amv(Mpeg4DecContext *ctx, int n)
         for (y = 0; y < 16; y++) {
             int v;
 
-            v = mb_v + dy * y;
+            v = mb_v + (unsigned)dy * y;
             // FIXME optimize
             for (x = 0; x < 16; x++) {
                 sum += v >> shift;