sockstat(1): tolerate situation where file info cannot be fetched

Either due to a race, or to the privilege restrictions, it is not
guaranteed that kern.files returned file information for all pcbs
read from net.inet.<proto>.pcblist. In this case the file rbtree does
not return the matching file by data address, and code must avoid
dereferencing NULL.

PR: 279875
Reviewed by: asomers
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D46050