libbegemot: Disable -Wuse-after-free.

The _xrealloc() function prints pointer values for internal assertion
failures and in one case does so after it has freed the pointer.

Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D36819

lib/libbegemot/Makefile

@@ -25,3 +25,5 @@ MLINKS=	rpoll.3 poll_register.3		\
 	rpoll.3 poll_dispatch.3
 
 .include <bsd.lib.mk>
+
+CWARNFLAGS+=	${NO_WUSE_AFTER_FREE}