Fix saving STRUCTURED-DATA in RFC 5424 log format

When saving log files in RFC 5424 format presence of STRUCTURED-DATA
in message caused line to not appear in log file at all, because
a variable forgotten to initialize. This could be tested by (slightly
modified) example from RFC:

echo '<165>1 2003-10-11T22:14:15.003Z mymachine.example.com su 12345 ID47 [exampleSDID@32473 iut="3" eventSource="Application" eventID="1011"][id@2 test="tast"] BOM"su root" failed for lonvick on /dev/pts/8" ' | nc -w1 -Uu /var/run/log

While here, update stale information in comment to logmsg() that RFC 5424
log format was not supported.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/717