avcodec/xvididct: Fix integer overflow in MULT()

Fixes: signed integer overflow: 23170 * 95058 cannot be represented in type 'int'
Fixes: 20295/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MJPEG_fuzzer-5800212870463488

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

libavcodec/xvididct.c

@@ -142,7 +142,7 @@ static int idct_row(short *in, const int *const tab, int rnd)
 #define TAN3  0xAB0E
 #define SQRT2 0x5A82
 
-#define MULT(c, x, n)  (((c) * (x)) >> (n))
+#define MULT(c, x, n)  ((unsigned)((int)((c) * (unsigned)(x)) >> (n)))
 // 12b version => #define MULT(c,x, n)  ((((c) >> 3) * (x)) >> ((n) - 3))
 // 12b zero-testing version: