Fixed `ByteStream_S::WriteByte: Overflow!` and `Network_WriteBuffer: Overflow!` messages during start up

These error messages went unnoticed to me from earlier testing of the login improvements. cl_autologin was missing a check to ensure that the user was also in an online game before attempting to request a login. Also, since neither cl_autologin nor login_default_user need to do anything during start up, their callbacks shouldn't be executed.