[gtk] compare-render: Assert that no error has been set

[Timm Bäder <baedert src gnome org>]

commit 74ab3cdf75f1c3eadd64432c19230543d6de45ff
Author: Timm Bäder <mail baedert org>
Date:   Sat Feb 23 07:57:12 2019 +0100

    compare-render: Assert that no error has been set
    
    Much better error message than just the one from the node != NULL
    assertion in the next line.

 testsuite/gsk/compare-render.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/testsuite/gsk/compare-render.c b/testsuite/gsk/compare-render.c
index 01b26c9bf2..5081daac23 100644
--- a/testsuite/gsk/compare-render.c
+++ b/testsuite/gsk/compare-render.c
@@ -104,6 +104,7 @@ main (int argc, char **argv)
     node = gsk_render_node_deserialize (bytes, &error);
     g_bytes_unref (bytes);
 
+    g_assert_no_error (error);
     g_assert (node != NULL);
   }