[vala/0.40] libvaladoc/gtkdoc-importer: Correctly retrieve "url" from "ulink" elements

[Rico Tzschichholz <ricotz src gnome org>]

commit e30ae6c60b5a0110a3e1df01c01f9468ca3c4ae3
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Thu Aug 27 10:02:19 2020 +0200

    libvaladoc/gtkdoc-importer: Correctly retrieve "url" from "ulink" elements
    
    This caused criticals like:
    
      g_uri_parse_scheme: assertion 'uri != NULL' failed

 libvaladoc/documentation/gtkdoccommentparser.vala | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/libvaladoc/documentation/gtkdoccommentparser.vala 
b/libvaladoc/documentation/gtkdoccommentparser.vala
index ede991ce8..bf3be801a 100644
--- a/libvaladoc/documentation/gtkdoccommentparser.vala
+++ b/libvaladoc/documentation/gtkdoccommentparser.vala
@@ -358,7 +358,12 @@ public class Valadoc.Gtkdoc.Parser : Object, ResourceLocator {
                }
 
                StringBuilder builder = new StringBuilder ();
-               string url = current.attributes.get ("linkend");
+               string url;
+               if (is_internal) {
+                       url = current.attributes.get ("linkend");
+               } else {
+                       url = current.attributes.get ("url");
+               }
                next ();
 
                // TODO: check xml