EVFILT_SIGNAL: do not use target process pointer on detach
PR: 275286
Approved by: so
Security: FreeBSD-EN-24:03.kqueue
(cherry picked from commit ed410b78edc53e17b5a3e93ace2adbeb3a734ae9)
(cherry picked from commit 55e91944998c128d74b94b9b48a04ef41ff5e9d0)