xsysconf: Only fail on error results and errno set

When testing nptl/tst-pthread-attr-affinity-fail fails with:

    error: xsysconf.c:33: sysconf (83): Cannot allocate memory
    error: 1 test failures

This happens as xsysconf checks the errno after running sysconf.
Internally the sysconf request for _SC_NPROCESSORS_CONF on linux
allocates memory.  But there is a problem, even though malloc succeeds
errno is getting set to ENOMEM.

POSIX allows successful calls to clobber errno.  So xsysconf just
checking errno is wrong.  Fix xsysconf by only failing if we have an
error result and errno is set.