Don't pass null variables to snprintf.

[ddpbsd]

New logging from OpenBSD alerted me to the problem:

Sep  1 13:48:12 ix ossec-dbd: vfprintf %s NULL in "INSERT INTO alert(server_id,rule_id,level,timestamp,location_id,src_ip,src_port,dst_ip,dst_port,alertid,"user",full_log) VALUES ('%u', '%u','%u','%u', '%u', '%s', '%u', '%s', '%u', '%s', '%s', '%s')"
Sep  1 13:51:37 ix ossec-dbd: vfprintf %s NULL in "%s: DEBUG: Connecting to '%s', using '%s', '%s', '%s', %d,'%s'."
Sep  1 13:51:42 ix ossec-dbd: vfprintf %s NULL in "REPLACE INTO signature(rule_id, level, description) VALUES ('%u','%u','%s')"
Sep  1 13:53:01 ix ossec-dbd: vfprintf %s NULL in "REPLACE INTO signature(rule_id, level, description) VALUES ('%u','%u','%s')"

I'm not entirely sure it's all fixed yet, but putting this here so I at least don't forget about it.

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This change is 

[jrossi]

This looks like just a formatting change.