bsdinstall partedit: Use snprintf instead of sprintf for error messages

When generating a message for a dialog box into a static buffer, use
snprintf instead of sprintf to avoid buffer overflows.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42240

(cherry picked from commit 6e8bf24073a1867c4b21bd84438a8b01ce91d82d)