Coverity: Forward NULL: CID 59, 58, 56

Coverity: Forward NULL: CID 59, 58, 56

	* src/ui/lwlib/lwlib-Xaw.c (xaw_generic_callback): Make sure you
	do not dereference possible NULL val.

	* src/ui/lwlib/lwlib-Xaw.c (wm_delete_window): Make sure widget
	and kids are only used when valid.

	* src/ui/lwlib/lwlib.c (replace_widget_value_tree): Make sure node
	is only used if != NULL.

	* src/ui/lwlib/lwlib.c (lw_map_widget_values): Make sure info is
	only used if != NULL.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>