[libc++] Increase portability of xalloc test

Do not assume that xalloc() starts at 0, which is not specified by the
Standard.

Thanks to Andrey Maksimov for the patch.
Differential Revision: https://reviews.llvm.org/D58299

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@355160 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed