Replace mistaken test of memtable size before last flush

kv.go

@@ -361,7 +361,7 @@ func (s *KV) Close() (err error) {
 	// and remove them completely, while the block / memtable writer is still
 	// trying to push stuff into the memtable. This will also resolve the value
 	// offset problem: as we push into memtable, we update value offsets there.
-	if s.mt.Size() > 0 {
+	if !s.mt.Empty() {
 		s.elog.Printf("Flushing memtable")
 		for {
 			pushedFlushTask := func() bool {

skl/skl.go

@@ -345,6 +345,11 @@ func (s *Skiplist) Put(key []byte, v y.ValueStruct) {
 	}
 }
 
+// Empty returns if the Skiplist is empty.
+func (s *Skiplist) Empty() bool {
+	return s.findLast() == nil
+}
+
 // findLast returns the last element. If head (empty list), we return nil. All the find functions
 // will NEVER return the head nodes.
 func (s *Skiplist) findLast() *node {