swscale/swscale: fix srcStride/srcSlice typo

Fixes part of Ticket3466
Found by: Andrey_Karpov / PVS-Studio
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/swscale.c

@@ -929,7 +929,7 @@ int attribute_align_arg sws_scale(struct SwsContext *c,
     uint8_t *dst2[4];
     uint8_t *rgb0_tmp = NULL;
 
-    if (!srcSlice || !dstStride || !dst || !srcSlice) {
+    if (!srcStride || !dstStride || !dst || !srcSlice) {
         av_log(c, AV_LOG_ERROR, "One of the input parameters to sws_scale() is NULL, please check the calling code\n");
         return 0;
     }