pgsql: Error out if waiting on socket readiness without a specified soc

Error out if waiting on socket readiness without a specified socket.

Previously we just ignored such an attempt, but that seems to serve no
purpose but making things harder to debug.

Discussion: 20160114143931.GG10941@awork2.anarazel.de
    20151230173734.hx7jj2fnwyljfqek@alap3.anarazel.de
Reviewed-By: Robert Haas

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6bc4d95fcc2a432fc202cba03d5393be096f0422

Modified Files
--------------
src/backend/port/unix_latch.c  | 9 +++++----
src/backend/port/win32_latch.c | 9 +++++----
2 files changed, 10 insertions(+), 8 deletions(-)