rcon server: add an explicit check against empty password, instead of assuming the...

rcon server: add an explicit check against empty password, instead of assuming the calling code is correct.

This logic had a bug in the past that had been introduced by r8886 and
accidentally fixed when introducing multi-account support in r9420. The bug was
that rcon_password being empty led to the empty password being accepted in some
cases, as there was never any explicit logic to handle that in rcon - it was
more "accidentally" rejected by a packet formatting check which does not apply
to srcon as the empty password wouldn't be empty over the wire.

In other words: DP builds from [r8886, r9419] had a serious vulnerability and
the only workaround is to always have rcon_password set.

The check added here will complain and reject in case the calling code for some
reason no longer prevents empty passwords from being accepted.

Also adding comments to the new logic explaining how it prevents the empty
password from being accepted.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12334 d7cf8633-e32d-0410-b094-e92efae38249