avcodec/diracdec: Prevent integer overflow in intermediate in global_mv()

Fixes: signed integer overflow: -393471 * 5460 cannot be represented in type 'int'
Fixes: 8890/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-6299775379963904

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5129040646)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/diracdec.c

@@ -1186,8 +1186,8 @@ static void global_mv(DiracContext *s, DiracBlock *block, int x, int y, int ref)
     int *c      = s->globalmc[ref].perspective;
 
     int m       = (1<<ep) - (c[0]*x + c[1]*y);
-    int64_t mx  = m * (int64_t)((A[0][0] * x + A[0][1]*y) + (1<<ez) * b[0]);
-    int64_t my  = m * (int64_t)((A[1][0] * x + A[1][1]*y) + (1<<ez) * b[1]);
+    int64_t mx  = m * (int64_t)((A[0][0] * (int64_t)x + A[0][1]*(int64_t)y) + (1<<ez) * b[0]);
+    int64_t my  = m * (int64_t)((A[1][0] * (int64_t)x + A[1][1]*(int64_t)y) + (1<<ez) * b[1]);
 
     block->u.mv[ref][0] = (mx + (1<<(ez+ep))) >> (ez+ep);
     block->u.mv[ref][1] = (my + (1<<(ez+ep))) >> (ez+ep);