PadMessageHandler: Fix stats null dereference
PadMessageHandler: Fix stats null dereference

It is possible for the stats to be read before the
`expressCreateServer` hook is called (in particular: when there is an
error during startup), which is when the `socketio` variable is set.
Check for non-null `socketio` before attempting to count the number of
socket.io connections.

ether/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite