Add missing check for wal_debug GUC.

9a20a9b2 added a new elog(), enabled when WAL_DEBUG is defined. The
other WAL_DEBUG dependant messages check for the wal_debug GUC, but this
one did not. While at it replace 'upto' with 'up to'.

Discussion: 20150610110253.GF3832@alap3.anarazel.de

Backpatch to 9.4, the first release containing 9a20a9b2.

src/backend/access/transam/xlog.c

@@ -1983,9 +1983,9 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
 	LWLockRelease(WALBufMappingLock);
 
 #ifdef WAL_DEBUG
-	if (npages > 0)
+	if (XLOG_DEBUG && npages > 0)
 	{
-		elog(DEBUG1, "initialized %d pages, upto %X/%X",
+		elog(DEBUG1, "initialized %d pages, up to %X/%X",
 			 npages, (uint32) (NewPageEndPtr >> 32), (uint32) NewPageEndPtr);
 	}
 #endif