intrng: move check for existing event to isrc_event_create()

[ehem_freebsd_m5p.com]

Summary

With a single caller it doesn't matter whether the check is in
isrc_event_create() or isrc_add_handler(). If another caller was added,
both callers would need the check. As such move the check to where it
can be shared.

[ehem_freebsd_m5p.com]

Ping D38234.