Fix incorrect assertion bound in WaitForLSN()

The assertion checking MyProcNumber used MaxBackends as the upper
bound, but the procInfos array is allocated with size
MaxBackends + NUM_AUXILIARY_PROCS. This inconsistency would cause
a false assertion failure if an auxiliary process calls WaitForLSN().

Author: Xuneng Zhou <xunengzhou@gmail.com>

src/backend/access/transam/xlogwait.c

@@ -321,7 +321,7 @@ WaitForLSN(WaitLSNType lsnType, XLogRecPtr targetLSN, int64 timeout)
 	Assert(waitLSNState);
 
 	/* Should have a valid proc number */
-	Assert(MyProcNumber >= 0 && MyProcNumber < MaxBackends);
+	Assert(MyProcNumber >= 0 && MyProcNumber < MaxBackends + NUM_AUXILIARY_PROCS);
 
 	if (timeout > 0)
 	{