avutil/mathematics: Fix division by 0

Fixes: CID1341571
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/mathematics.c

@@ -85,7 +85,7 @@ int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding rnd)
         else {
             int64_t ad = a / c;
             int64_t a2 = (a % c * b + r) / c;
-            if (ad >= INT32_MAX && ad > (INT64_MAX - a2) / b)
+            if (ad >= INT32_MAX && b && ad > (INT64_MAX - a2) / b)
                 return INT64_MIN;
             return ad * b + a2;
         }