sort: Fix random sort

bwsrawdata() is supposed to return the string buffer.

PR: 259451
Reported by: sigsys@gmail.com
Fixes: d053fb22f6d3 ("usr.bin/sort: Avoid UBSan errors")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e9bfb50d5e7aa5d673a5a35318820320c4190d33)