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.

[Install]
WantedBy=sockets.target

[Socket]
ListenStream=0.0.0.0:5222
ListenStream=5222
BindIPv6Only=ipv6-only
Accept=false
Service=prosody.service