cifs: fix incorrect check for null pointer in header_assemble

Although very unlikely that the tlink pointer would be null in this case,
get_next_mid function can in theory return null (but not an error)
so need to check for null (not for IS_ERR, which can not be returned
here).

Address warning:

        fs/smbfs_client/connect.c:2392 cifs_match_super()
        warn: 'tlink' isn't an ERR_PTR

Pointed out by Dan Carpenter via smatch code analysis tool

CC: stable@vger.kernel.org
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -2389,9 +2389,10 @@ cifs_match_super(struct super_block *sb, void *data)
 	spin_lock(&cifs_tcp_ses_lock);
 	cifs_sb = CIFS_SB(sb);
 	tlink = cifs_get_tlink(cifs_sb_master_tlink(cifs_sb));
-	if (IS_ERR(tlink)) {
+	if (tlink == NULL) {
+		/* can not match superblock if tlink were ever null */
 		spin_unlock(&cifs_tcp_ses_lock);
-		return rc;
+		return 0;
 	}
 	tcon = tlink_tcon(tlink);
 	ses = tcon->ses;