adjust memory layout to avoid possible nil pointer

weed/storage/needle_map.go

@@ -37,10 +37,10 @@ type NeedleMapper interface {
 }
 
 type baseNeedleMapper struct {
+	mapMetric
+
 	indexFile           *os.File
 	indexFileAccessLock sync.Mutex
-
-	mapMetric
 }
 
 func (nm *baseNeedleMapper) IndexFileSize() uint64 {

weed/storage/needle_map_memory.go

@@ -10,9 +10,8 @@ import (
 )
 
 type NeedleMap struct {
-	m needle_map.NeedleValueMap
-
 	baseNeedleMapper
+	m needle_map.NeedleValueMap
 }
 
 func NewCompactNeedleMap(file *os.File) *NeedleMap {