ntp: NULL pointer deref when create_interface() fails

Fix NULL pointer dereference when create_interface() fails in
update_interfaces(). The upstream bug report says a typo was introduced
by https://bk.ntp.org/ntp-dev/ntpd/ntp_io.c?PAGE=diffs&REV=66175036PETA6g__fON8oNrjL54Ttw. ep should have been ep2.

Fix obtained from upstream bug report.

PR: 285065
Upstream bug: https://bugs.ntp.org/show_bug.cgi?id=3939
MFC after: 1 minute