swscale: fix another integer overflow.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libswscale/utils.c

@@ -1014,7 +1014,7 @@ int sws_init_context(SwsContext *c, SwsFilter *srcFilter, SwsFilter *dstFilter)
     c->vLumBufSize= c->vLumFilterSize;
     c->vChrBufSize= c->vChrFilterSize;
     for (i=0; i<dstH; i++) {
-        int chrI= i*c->chrDstH / dstH;
+        int chrI = (int64_t) i * c->chrDstH / dstH;
         int nextSlice= FFMAX(c->vLumFilterPos[i   ] + c->vLumFilterSize - 1,
                            ((c->vChrFilterPos[chrI] + c->vChrFilterSize - 1)<<c->chrSrcVSubSample));