libgimp, libgimpconfig: gimp_config_deserialize_file() should not…

… assert the existence of GError.

This is even worse as deserialize() method does not even take a GError
parameter anyway so this assert will always go off when a
deserialization failed (which happened in my case as I was working on a
plug-in API, hence gimp_procedure_config_load_last() actually failed to
load a previous version of the plug-in-settings when I changed a
procedure arg's type).

Just fail the deserialization normally and let the calling code handling
this case. Nevertheless it is kind of useful to bubble-up the error to
calling code, so I add a TODO in the interface header (hopefully to see
and improve this before we release GIMP 3.0).

libgimp/gimpprocedureconfig.c

@@ -981,7 +981,8 @@ gimp_procedure_config_load_last (GimpProcedureConfig  *config,
                                           file,
                                           NULL, error);
 
-  if (! success && (*error)->code == GIMP_CONFIG_ERROR_OPEN_ENOENT)
+  if (! success && error && *error &&
+      (*error)->code == GIMP_CONFIG_ERROR_OPEN_ENOENT)
     {
       g_clear_error (error);
     }

libgimpconfig/gimpconfig-iface.c

@@ -494,9 +494,6 @@ gimp_config_deserialize_file (GimpConfig  *config,
 
   gimp_scanner_unref (scanner);
 
-  if (! success)
-    g_assert (error == NULL || *error != NULL);
-
   return success;
 }
 

libgimpconfig/gimpconfig-iface.h

@@ -40,6 +40,9 @@ struct _GimpConfigInterface
   gboolean     (* serialize)            (GimpConfig       *config,
                                          GimpConfigWriter *writer,
                                          gpointer          data);
+  /* TODO: we should add a GError** parameter to the deserialize()
+   * method in order to be able to report errors.
+   */
   gboolean     (* deserialize)          (GimpConfig       *config,
                                          GScanner         *scanner,
                                          gint              nest_level,