swscale: Fix an undefined behaviour

Prevent a division by zero down the codepath.

Sample-Id: 00001721-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libswscale/utils.c

@@ -331,7 +331,7 @@ static av_cold int initFilter(int16_t **outFilter, int32_t **filterPos,
 
         xDstInSrc = xInc - 0x10000;
         for (i = 0; i < dstW; i++) {
-            int xx = (xDstInSrc - ((filterSize - 2) << 16)) / (1 << 17);
+            int xx = (xDstInSrc - ((int64_t)(filterSize - 2) << 16)) / (1 << 17);
             int j;
             (*filterPos)[i] = xx;
             for (j = 0; j < filterSize; j++) {