Arrange that log_packet() isn't called for raw data logging if logctx is null.

Arrange that log_packet() isn't called for raw data logging if logctx is null.
This allows us to send data in ssh_init(), albeit at the expense of its not
being properly logged, so arrange to send the version string then if that's
sensible, which should reduce the number of round-trips required to bring
up an SSH-2 connection.

[originally from svn r7646]