Ping timeout server crash fix

By default, the checkPingTimeouts function works something like this:
```
for user in users:
    if user pinged out:
        delete user
    send user a ping
```
This has the unfortunate effect of trying to send users who have pinged out a ping message, which crashes the server when there (was) only one user in the list, since it sees user 0 is pinged out, deletes user 0, and then tries to send user 0 a ping message.

This PR basically just changes it to the following (so we don't send non-existent users a ping and crash the server with an array out-of-bounds error):
```
for user in users:
    if user pinged out:
        delete user
    else:
        send user a ping
```