mod_c2s_limit_sessions: Fix check when it's the first connection Creation of the sessions[username] entry happens after the "pre-resource-bind" event, before the "resource-bind" event. Thus, for a users first connection, it may not exist. This led to an 'attempt to index a nil value' error, fixed here.

This module produces more detailed HTTP logs for Prosodys built-in HTTP
server. The format is similar to that of Apache and go into Prosodys
normal logs at the `info` level.