Avoid mixing void and integer in a conditional expression.

The C standard says that the second and third arguments of a
conditional operator shall be both void type or both not-void
type.  The Windows version of INTERRUPTS_PENDING_CONDITION()
got this wrong.  It's pretty harmless because the result of
the operator is ignored anyway, but apparently recent versions
of MSVC have started issuing a warning about it.  Silence the
warning by casting the dummy zero to void.

Reported-by: Christian Ullrich <chris@chrullrich.net>
Author: Bryan Green <dbryan.green@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/cc4ef8db-f8dc-4347-8a22-e7ebf44c0308@chrullrich.net
Backpatch-through: 13

Author: tglsfdc

src/include/miscadmin.h

@@ -114,7 +114,8 @@ extern void ProcessInterrupts(void);
 	(unlikely(InterruptPending))
 #else
 #define INTERRUPTS_PENDING_CONDITION() \
-	(unlikely(UNBLOCKED_SIGNAL_QUEUE()) ? pgwin32_dispatch_queued_signals() : 0, \
+	(unlikely(UNBLOCKED_SIGNAL_QUEUE()) ? \
+	 pgwin32_dispatch_queued_signals() : (void) 0, \
 	 unlikely(InterruptPending))
 #endif