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

[kib]

Summary

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