new-calendar-page: improve URI validity check

This check passes if the URI is empty. Normally this callback only runs
when there is text in the URL entry, but there is a race window where
the user could delete it first. (Passing NULL to a SoupURI creates an
invalid non-NULL empty URI that can be filled in later.)

src/gui/calendar-management/gcal-new-calendar-page.c

@@ -341,7 +341,7 @@ validate_url_cb (gpointer data)
   self->validate_url_resource_id = 0;
 
   uri = soup_uri_new (gtk_entry_get_text (self->calendar_address_entry));
-  valid_uri = uri != NULL;
+  valid_uri = uri != NULL && SOUP_URI_IS_VALID (uri);
 
   set_url_entry_error (self, !valid_uri);