avcodec/dcadsp: Fix runtime error: signed integer overflow

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 9244b839b7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dcadsp.c

@@ -320,7 +320,7 @@ static void dmix_sub_c(int32_t *dst, const int32_t *src, int coeff, ptrdiff_t le
     int i;
 
     for (i = 0; i < len; i++)
-        dst[i] -= mul15(src[i], coeff);
+        dst[i] -= (unsigned)mul15(src[i], coeff);
 }
 
 static void dmix_add_c(int32_t *dst, const int32_t *src, int coeff, ptrdiff_t len)