Update file_storage.go

if the first if statement is true, fn[7]=='.' ,fn[7:] is a start with '.' slice. ParseUint will return an error.
diff --git a/leveldb/storage/file_storage.go b/leveldb/storage/file_storage.go
index 75439f6..46cc9d0 100644
--- a/leveldb/storage/file_storage.go
+++ b/leveldb/storage/file_storage.go
@@ -221,7 +221,7 @@
 					fs.log(fmt.Sprintf("skipping %s: invalid file name", fn))
 					continue
 				}
-				if _, e1 := strconv.ParseUint(fn[7:], 10, 0); e1 != nil {
+				if _, e1 := strconv.ParseUint(fn[8:], 10, 0); e1 != nil {
 					fs.log(fmt.Sprintf("skipping %s: invalid file num: %v", fn, e1))
 					continue
 				}