swscale: fix ticket #4877

(cherry picked from commit a8602dde)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/swscale.c

@@ -476,7 +476,7 @@ static int swscale(SwsContext *c, const uint8_t *src[],
     ff_init_slice_from_src(vout_slice, (uint8_t**)dst, dstStride, c->dstW,
             dstY, dstH, dstY >> c->chrDstVSubSample,
             FF_CEIL_RSHIFT(dstH, c->chrDstVSubSample));
-
+    if (srcSliceY == 0) {
         hout_slice->plane[0].sliceY = lastInLumBuf + 1;
         hout_slice->plane[1].sliceY = lastInChrBuf + 1;
         hout_slice->plane[2].sliceY = lastInChrBuf + 1;
@@ -487,6 +487,7 @@ static int swscale(SwsContext *c, const uint8_t *src[],
         hout_slice->plane[2].sliceH =
         hout_slice->plane[3].sliceH = 0;
         hout_slice->width = dstW;
+    }
 #endif
 
     for (; dstY < dstH; dstY++) {