Fix: NULL pointer check in ssh_channel_is_closed

The ssh_channel_is_closed function would crash when
accessing channel->session->alive if session is NULL.
This patch adds a null check before accessing the session
pointer.

- build succeeded
- unit test passed
- no new unit test added

libssh/libssh-mirror#239



Signed-off-by: Raviraaja Lakshmanan <mailstoraviraaja@gmail.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>