Fix possible leak of semaphore count.

Commit 4aec49899e reorganized the order
of operations here so that we no longer increment the number of "extra
waits" before locking the semaphore, but it did not change the
starting value of extraWaits from 0 to -1 to compensate.  In the worst
case, this could leak a semaphore count, but that seems to be unlikely
in practice.

Discussion: http://postgr.es/m/CAA4eK1JyVqXiMba+-a589Rk0pyHsyKkGxeumVKjU6Y74hdrVLQ@mail.gmail.com

Amit Kapila, per an off-list report by Dilip Kumar.  Reviewed by me.

src/backend/storage/ipc/procarray.c

@@ -484,7 +484,6 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	volatile PROC_HDR *procglobal = ProcGlobal;
 	uint32		nextidx;
 	uint32		wakeidx;
-	int			extraWaits = -1;
 
 	/* We should definitely have an XID to clear. */
 	Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
@@ -511,6 +510,8 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	 */
 	if (nextidx != INVALID_PGPROCNO)
 	{
+		int			extraWaits = 0;
+
 		/* Sleep until the leader clears our XID. */
 		for (;;)
 		{