fd leak catched by gcc-analyzer

As said in .gcc-analyzer-false-positives, I'm not really in the mood to fix this, but it should be fixed:

../libmailwatch-core/mailwatch-net-conn.c: In function ‘xfce_mailwatch_net_conn_do_connect’:
../libmailwatch-core/mailwatch-net-conn.c:171:45: warning: leak of file descriptor ‘*net_conn.fd’ [CWE-775] [-Wanalyzer-fd-leak]
  171 |             && !TIMER_EXPIRED(RECV_TIMEOUT) && SHOULD_CONTINUE(net_conn));
      |                                             ^
  ‘xfce_mailwatch_net_conn_do_connect’: event 1
  171 |             && !TIMER_EXPIRED(RECV_TIMEOUT) && SHOULD_CONTINUE(net_conn));
      |                                             ^
      |                                             |
      |                                             (1) ⚠️  ‘*net_conn.fd’ leaks here