avcodec/vp3dsp: Fix multiple signed integer overflow: 46341 * 47523 cannot be represented in type 'int'

Fixes: 664/clusterfuzz-testcase-4917047475568640

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

libavcodec/vp3dsp.c

@@ -41,7 +41,7 @@
 #define xC6S2 25080
 #define xC7S1 12785
 
-#define M(a, b) (((a) * (b)) >> 16)
+#define M(a, b) ((int)((SUINT)(a) * (b)) >> 16)
 
 static av_always_inline void idct(uint8_t *dst, int stride,
                                   int16_t *input, int type)