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 23078689a9 did so.

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

src/backend/port/win32/signal.c

@@ -52,6 +52,7 @@ static BOOL WINAPI pg_console_handler(DWORD dwCtrlType);
 void
 pg_usleep(long microsec)
 {
+	Assert(pgwin32_signal_event != NULL);
 	if (WaitForSingleObject(pgwin32_signal_event,
 							(microsec < 500 ? 1 : (microsec + 500) / 1000))
 		== WAIT_OBJECT_0)
@@ -108,6 +109,7 @@ pgwin32_dispatch_queued_signals(void)
 {
 	int			exec_mask;
 
+	Assert(pgwin32_signal_event != NULL);
 	EnterCriticalSection(&pg_signal_crit_sec);
 	while ((exec_mask = UNBLOCKED_SIGNAL_QUEUE()) != 0)
 	{
@@ -209,6 +211,7 @@ pgwin32_create_signal_listener(pid_t pid)
 void
 pg_queue_signal(int signum)
 {
+	Assert(pgwin32_signal_event != NULL);
 	if (signum >= PG_SIGNAL_COUNT || signum <= 0)
 		return;
 

src/port/open.c

@@ -70,6 +70,9 @@ pgwin32_open(const char *fileName, int fileFlags,...)
 						 (O_RANDOM | O_SEQUENTIAL | O_TEMPORARY) |
 						 _O_SHORT_LIVED | O_DSYNC | O_DIRECT |
 						 (O_CREAT | O_TRUNC | O_EXCL) | (O_TEXT | O_BINARY))) == fileFlags);
+#ifndef FRONTEND
+	Assert(pgwin32_signal_event != NULL);	/* small chance of pg_usleep() */
+#endif
 
 #ifdef FRONTEND