Fix bug where pg_receivexlog goes into busy loop if -s option is set to 0.

The problem is that pg_receivexlog calls select(2) with timeout=0 and
goes into busy loop when --status-interval option is set to 0. This bug
was introduced by the commit,
74cbe966fe2d76de1d607d933c98c144dab58769.

Per report from Sawada Masahiko