swscale/tests/swscale: Initialize res to a non random error code

Regression since: 3adffab073

-1 is consistent what other error paths return

Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/tests/swscale.c

@@ -423,8 +423,10 @@ bad_option:
         for (x = 0; x < W * 4; x++)
             rgb_data[ x + y * 4 * W] = av_lfg_get(&rand);
     res = sws_scale(sws, rgb_src, rgb_stride, 0, H / 12, (uint8_t * const *) src, stride);
-    if (res < 0 || res != H)
+    if (res < 0 || res != H) {
+        res = -1;
         goto error;
+    }
     sws_freeContext(sws);
     av_free(rgb_data);