Correct pg_recvlogical server version test.

The predecessor test boiled down to "PQserverVersion(NULL) >= 100000",
which is always false.  No release includes that, so it could not have
reintroduced CVE-2018-1058.  Back-patch to 9.4, like the addition of the
predecessor in commit 8d2814f2.

Discussion: https://postgr.es/m/20180422215551.GB2676194@rfd.leadboat.com