cifs: throw a warning if negotiate or sess_setup ops are passed NULL server or session pointers

These look pretty cargo-culty to me, but let's be certain. Leave
them in place for now. Pop a WARN if it ever does happen. Also,
move to a more standard idiom for setting the "server" pointer.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/sess.c

@@ -576,8 +576,10 @@ CIFS_SessSetup(const unsigned int xid, struct cifs_ses *ses,
 	u16 blob_len;
 	char *ntlmsspblob = NULL;
 
-	if (ses == NULL)
+	if (ses == NULL) {
+		WARN(1, "%s: ses == NULL!", __func__);
 		return -EINVAL;
+	}
 
 	type = ses->server->secType;
 	cifs_dbg(FYI, "sess setup type %d\n", type);