pgsql: Assert that pgwin32_signal_initialize() has been called earlyen

Assert that pgwin32_signal_initialize() has been called early enough.

Before the pgwin32_signal_initialize() call, the backend version of
pg_usleep() has no effect.  No in-tree code falls afoul of that today,
but temporary commit 23078689a9921968ac0873b017be6e7f772f10bc did so.

Discussion: https://postgr.es/m/20190402135442.GA1173872@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ab9ed9be2378f02edf613e0543a29287e7484338

Modified Files
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src/backend/port/win32/signal.c | 3 +++
src/port/open.c                 | 3 +++
2 files changed, 6 insertions(+)