pgsql: Fix latent portability issue in pgwin32_dispatch_queued_signals(

Fix latent portability issue in pgwin32_dispatch_queued_signals().

The first iteration of the signal-checking loop would compute sigmask(0)
which expands to 1<<(-1) which is undefined behavior according to the
C standard. The lack of field reports of trouble suggest that it
evaluates to 0 on all existing Windows compilers, but that's hardly
something to rely on. Since signal 0 isn't a queueable signal anyway,
we can just make the loop iterate from 1 instead, and save a few cycles
as well as avoiding the undefined behavior.

In passing, avoid evaluating the volatile expression UNBLOCKED_SIGNAL_QUEUE
twice in a row; there's no reason to waste cycles like that.

Noted by Aleksander Alekseev, though this isn't his proposed fix.
Back-patch to all supported branches.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/43b73d1a40463ff1fe7dee4d61a46a158629ab2b

Modified Files
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src/backend/port/win32/signal.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)