stand: use snprintf here

This code was written prior to snprintf being in the then libstand (now
libsa). Since we have it, use it for extra safety. The code already
tries to be safe, but since we have snprintf as well, the added layer of
protection will suffice. The current code reserves 16 bytes (plus a NUL)
at the end for worst case of inet_ntoa, which is still a little
pessimal, but safe from overflow.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35102