postgres_fdw: Useless if-test in GetConnection()

Hi,

While working on something else, I noticed that the “if (entry->conn
== NULL)” test after doing disconnect_pg_server() when re-establishing
a given connection in GetConnection() is pointless, because the former
function ensures that entry->conn is NULL.  So I removed the if-test.
Attached is a patch for that.  I think we could instead add an
assertion, but I did not, because we already have it in
make_new_connection().

This would be harmless, so I am planning to apply the patch to HEAD only.

Best regards,
Etsuro Fujita