Remove the (no longer effective) empty if-branch.

Remove the (no longer effective) empty if-branch.

	* glib/gmessages.h (g_assert):
	(g_return_if_fail):
	(g_return_val_if_fail): Remove the (no longer effective) empty
	if-branch.

	* glib/gmacros.h: Change the definition of G_LIKELY, so that
	g_return_if_fail() and friends still trigger a gcc warning if
	the expr is an assignment.