Fix bug in the test of file descriptor of current WAL file in pg_receivexlog.

In pg_receivexlog, in order to check whether the current WAL file is
being opened or not, its file descriptor has to be checked against -1
as an invalid value. But, oops, 7900e94 added the incorrect test
checking the descriptor against 1. This commit fixes that bug.

Back-patch to 9.4 where the bug was added.

Spotted by Magnus Hagander