avcodec/dcadsp: Fix integer overflow in dmix_add_c()

Fixes: signed integer overflow: 1515225320 + 759416059 cannot be represented in type 'int'
Fixes: 29256/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DCA_fuzzer-5719088561258496

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

libavcodec/dcadsp.c

@@ -329,7 +329,7 @@ static void dmix_add_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_scale_c(int32_t *dst, int scale, ptrdiff_t len)