security/sshguard: Fix memset() off-by-one

This bug causes a stack overflow (and crash due to failed stack check)
when certain IPv6 addresses are whitelisted on i386.

PR: 258179
Reported by: John Marshall <john@jmarshall.id.au>
MFH: 2021Q4

(cherry picked from commit c3381bf4d961159f4903f573c7f01fae85ad5a18)