fix index out of range when sid len = 1

add unit test for sess_file.go

session/sess_file.go

@@ -15,11 +15,11 @@
 package session
 
 import (
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"net/http"
 	"os"
-	"errors"
 	"path"
 	"path/filepath"
 	"strings"
@@ -180,6 +180,10 @@ func (fp *FileProvider) SessionExist(sid string) bool {
 	filepder.lock.Lock()
 	defer filepder.lock.Unlock()
 
+	if len(sid) < 2 {
+		return false
+	}
+
 	_, err := os.Stat(path.Join(fp.savePath, string(sid[0]), string(sid[1]), sid))
 	return err == nil
 }