Check if pthread_create(3) successfully created the thread prior to call
pthread_join(3). The variable tid is not yet initialized in case
the authentication fails at early stage, that would lead pthread_join be
called with an uninitialized variable.

CID: 1375950
Reported by: Coverity, cem
Reviewed by: cem
MFC after: 3 weeks.
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D11150