sws: move the early return for zero-sized slices higher up

Place it right after the input parameter validation. There is no point
in performing any setup if the sws_scale() call won't do anything.

libswscale/swscale.c

@@ -897,6 +897,10 @@ int attribute_align_arg sws_scale(struct SwsContext *c,
         return AVERROR(EINVAL);
     }
 
+    // do not mess up sliceDir if we have a "trailing" 0-size slice
+    if (srcSliceH == 0)
+        return 0;
+
     if (c->gamma_flag && c->cascaded_context[0])
         return scale_gamma(c, srcSlice, srcStride, srcSliceY, srcSliceH, dst, dstStride);
 
@@ -908,10 +912,6 @@ int attribute_align_arg sws_scale(struct SwsContext *c,
     memcpy(srcStride2, srcStride, sizeof(srcStride2));
     memcpy(dstStride2, dstStride, sizeof(dstStride2));
 
-    // do not mess up sliceDir if we have a "trailing" 0-size slice
-    if (srcSliceH == 0)
-        return 0;
-
     if (frame_start) {
         if (srcSliceY != 0 && srcSliceY + srcSliceH != c->srcH) {
             av_log(c, AV_LOG_ERROR, "Slices start in the middle!\n");