ctdb-scripts: Do not "correct" number of nfsd threads when it is 0

ctdb-scripts: Do not "correct" number of nfsd threads when it is 0

While 0 may indicate that all threads have exited after being stuck,
it may also indicate that nfsd should not be running due to being shut
down.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13860

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@samba.org>
Autobuild-User(master): Martin Schwenke <martins@samba.org>
Autobuild-Date(master): Sun Mar 31 11:47:44 UTC 2019 on sn-devel-144