kern_intr: Check for NULL event in intr_destroy()

It likely won't happen, but is consistent with the other functions of
this KPI.

Reviewed by: imp, jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D33479

(cherry picked from commit 39888ed7a3dca81c5d5c886e699dbcf58953137f)