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

(cherry picked from commit 996ee965976379f0455c5f59b1cf9c82bc2e5e1b)