[gimp] app: fix parsing of GUM cross-property references

[Ell <ell src gnome org>]

commit 8139836d5d406e0d79b8a57b288e8b2f98de4951
Author: Ell <ell_se yahoo com>
Date:   Mon Jun 1 22:39:11 2020 +0300

    app: fix parsing of GUM cross-property references

 app/propgui/gimppropgui-eval.c | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/app/propgui/gimppropgui-eval.c b/app/propgui/gimppropgui-eval.c
index 422578b13d..940a9a9eb0 100644
--- a/app/propgui/gimppropgui-eval.c
+++ b/app/propgui/gimppropgui-eval.c
@@ -926,6 +926,15 @@ gimp_prop_eval_parse_reference (GObject      *config,
       g_free (property_name);
 
       *ref_key = g_strdup (*t);
+
+      gimp_prop_eval_read_token (expr, t, error);
+
+      if (*error)
+        {
+          g_free (*ref_key);
+
+          return FALSE;
+        }
     }
 
   return TRUE;