mach: strerror must not return NULL (bug 30555)

This follows 1d44530a ("string: strerror must not return NULL (bug 30555)"):

«
    For strerror, this fixes commit 28aff047 ("string:
    Implement strerror in terms of strerror_l").  This commit avoids
    returning NULL for strerror_l as well, although POSIX allows this
    behavior for strerror_l.
»