fix duplicate PAM msgs, missing loginmsg reset

without this change in mm_answer_pam_account all messages added in
auth-pam.c sshpam_query(...) case PAM_SUCCESS end up sent here, then are
still sitting in the loginmsg buffer and printed a second time in
session.c do_login(...)

monitor.c

@@ -1123,6 +1123,7 @@ mm_answer_pam_account(struct ssh *ssh, int sock, struct sshbuf *m)
 	if ((r = sshbuf_put_u32(m, ret)) != 0 ||
 	    (r = sshbuf_put_stringb(m, loginmsg)) != 0)
 		fatal("%s: buffer error: %s", __func__, ssh_err(r));
+	sshbuf_reset(loginmsg);
 
 	mm_request_send(sock, MONITOR_ANS_PAM_ACCOUNT, m);