Do not return out of bound pointers from intr_lookup_source().

This hardens the code against driver and upper level bugs causing
invalid indexes used, e.g. on msi release.

Reported by: gallatin
Reviewed by: gallatin, hselasky
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D14470