clapic_handle_intr: KASSERT isrc != NULL

If an interrupt arrives at a CPU which isn't expecting that particular
vector, intr_lookup_source will return an isrc of NULL and we'll panic
when intr_execute_handlers increments *isrc->is_count.

Place a KASSERT a few nanoseconds earlier in order to leave some more
breadcrumbs for the next person to trip over this behaviour.

Tested on: EC2 r8i.96xlarge
MFC after: 3 weeks
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D55851