if: Remove unnecessary validation in the SIOCSIFNAME handler

A successful copyinstr() call guarantees that the returned string is
nul-terminated. Furthermore, the removed check would harmlessly compare
an uninitialized byte with '\0' if the new name is shorter than
IFNAMESIZ - 1.

Reported by: KMSAN
MFC after: 1 week
Sponsored by: The FreeBSD Foundation