swscale/swscale_unscaled: adjust last line copy

Fixes: out of array access
Fixes: DFVULN-694

*Reporter: Zhenpeng (Leo) Lin at depthfirst*

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

libswscale/swscale_unscaled.c

@@ -127,9 +127,13 @@ void ff_copyPlane(const uint8_t *src, int srcStride,
                   int srcSliceY, int srcSliceH, int width,
                   uint8_t *dst, int dstStride)
 {
+    if (!srcSliceH)
+        return;
+    av_assert0(srcSliceH > 0);
+
     dst += dstStride * srcSliceY;
     if (dstStride == srcStride && srcStride > 0) {
-        memcpy(dst, src, srcSliceH * dstStride);
+        memcpy(dst, src, (srcSliceH - 1) * dstStride + width);
     } else {
         int i;
         for (i = 0; i < srcSliceH; i++) {