MFC r315745:

MFC r315745:

Cherry-pick libcxxrt commit 8a853717e61d5d55cbdf74d9d0a7545da5d5ff92:

Author: David Chisnall <theraven@FreeBSD.org>
Date:   Wed Mar 22 12:27:08 2017 +0000

    Simplify some code.

    realloc() with a null pointer is equivalent to malloc, so we don't need
    to handle the two cases independently.

    Fixes #46

This should help with lang/beignet and other programs, which expect
__cxa_demangle(name, NULL, NULL, &status) to return zero in status.

PR:		213732

git-svn-id: svn://svn.freebsd.org/base/stable/10@315943 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f