smbd: uid: Don't crash if 'force group' is added to an existing share connection.

smbd: uid: Don't crash if 'force group' is added to an existing share connection.

smbd could crash if "force group" is added to a
share definition whilst an existing connection
to that share exists. In that case, don't change
the existing credentials for force group, only
do so for new connections.

Remove knownfail from regression test.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13690

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Fri Jan 25 16:31:27 CET 2019 on sn-devel-144

(cherry picked from commit e37f9956c1f2416408bad048a4618f6366086b6a)