Bug 794023 - Bad/Double free bugs found by scan-build.

In case of error in gimp_prop_eval_parse_reference(), we were obviously
freeing the string which had just been allocated by g_strdup(), not the
pointer to this string.
Thanks to Massimo for raising this issue.

app/propgui/gimppropgui-eval.c

@@ -882,7 +882,7 @@ gimp_prop_eval_parse_reference (GObject      *config,
 
   if (*error)
     {
-      g_free (ref_key);
+      g_free (*ref_key);
 
       return FALSE;
     }