avcodec/diracdec: Use int64 in global mv to prevent overflow

Fixes: runtime error: signed integer overflow: 361 * -6295541 cannot be represented in type 'int'
Fixes: 5911/clusterfuzz-testcase-minimized-6450382197751808

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 cbcbefdc3b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/diracdec.c

@@ -1393,8 +1393,8 @@ 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);
-    int mx      = m * ((A[0][0] * x + A[0][1]*y) + (1<<ez) * b[0]);
-    int my      = m * ((A[1][0] * x + A[1][1]*y) + (1<<ez) * b[1]);
+    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]);
 
     block->u.mv[ref][0] = (mx + (1<<(ez+ep))) >> (ez+ep);
     block->u.mv[ref][1] = (my + (1<<(ez+ep))) >> (ez+ep);