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