libbegemot: Disable -Wuse-after-free.

[jhb]

Summary

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

[arichardson]

Would saving the pointer address in a local uinptr_t/size_t fix the warning?

[arichardson]

Something like this: https://godbolt.org/z/xKxnheh5r