tevent: fix CID 1437974 dereference after null check

Probably a copy/paste error from the tevent_debug() statement a few
lines above as at this place we want to pass main_ev directly to
tevent_debug() anyway.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>

Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Tue Jul 17 16:21:21 CEST 2018 on sn-devel-144
