add missing checks to strlcpy() when copying envelope "destination" buffer

add missing checks to strlcpy() when copying envelope "destination" buffer
to the mda delivery buffer. we should never hit these unless we mistakenly
change the value of a define, better be safe than sorry.

(void) cast strlcpy/strlcat that cannot truncate or that we know and want
to truncate rather than lose (informative data not used by smtpd but
intended to help the human reading the log)