the statistical methord for fileCount should stay same (#3477)

the statistical methord for fileCount between should stay same
They all equal to the entry count in .idx file.
relate to commit(c7892bc)

weed/storage/needle_map_memory.go

@@ -35,8 +35,8 @@ func LoadCompactNeedleMap(file *os.File) (*NeedleMap, error) {
 func doLoading(file *os.File, nm *NeedleMap) (*NeedleMap, error) {
 	e := idx.WalkIndexFile(file, 0, func(key NeedleId, offset Offset, size Size) error {
 		nm.MaybeSetMaxFileKey(key)
+		nm.FileCounter++
 		if !offset.IsZero() && size.IsValid() {
-			nm.FileCounter++
 			nm.FileByteCounter = nm.FileByteCounter + uint64(size)
 			oldOffset, oldSize := nm.m.Set(NeedleId(key), offset, size)
 			if !oldOffset.IsZero() && oldSize.IsValid() {