Fix handling of invalid sockets returned by PQsocket()

Hi all,

After looking at Alvaro's message mentioning the handling of
PQsocket() for invalid sockets, I just had a look by curiosity at
other calls of this routine, and found a couple of issues:
1) In vacuumdb.c, init_slot() does not check for the return value of PQsocket():
slot->sock = PQsocket(conn);
2) In isolationtester.c, try_complete_step() should do the same.
3) In pg_recvlogical.c for StreamLogicalLog() I am spotting the same problem.
I guess those ones should be fixed as well, no?

The patch attached addresses those issues.

This has been raised in this message, but beginning a new thread makes
more sense:
http://www.postgresql.org/message-id/CAB7nPqTTZoiuVYGNonLVnZysStUSOfhKeO9FTrQbKWJ36UCdOA@mail.gmail.com
Regards,
--
Michael