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>

fs/smb/client/sess.c

@@ -76,7 +76,7 @@ cifs_ses_get_chan_index(struct cifs_ses *ses,
 	unsigned int i;
 
 	/* if the channel is waiting for termination */
-	if (server->terminate)
+	if (server && server->terminate)
 		return CIFS_INVAL_CHAN_INDEX;
 
 	for (i = 0; i < ses->chan_count; i++) {