smb3: add missing null server pointer check

Address static checker warning in cifs_ses_get_chan_index():
    warn: variable dereferenced before check 'server'
To be consistent, and reduce risk, we should add another check
for null server pointer.

Fixes: 88675b22 ("cifs: do not search for channel if server is terminating")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>