avoid using empty fileId to delete

fix https://github.com/chrislusf/seaweedfs/issues/183

go/filer/embedded_filer/files_in_leveldb.go

@@ -43,6 +43,9 @@ func (fl *FileListInLevelDb) CreateFile(dirId filer.DirectoryId, fileName string
 }
 func (fl *FileListInLevelDb) DeleteFile(dirId filer.DirectoryId, fileName string) (fid string, err error) {
 	if fid, err = fl.FindFile(dirId, fileName); err != nil {
+		if err == leveldb.ErrNotFound {
+			return "", nil
+		}
 		return
 	}
 	err = fl.db.Delete(genKey(dirId, fileName), nil)

go/weed/weed_server/filer_server_handlers.go

@@ -209,7 +209,7 @@ func (fs *FilerServer) DeleteHandler(w http.ResponseWriter, r *http.Request) {
 		err = fs.filer.DeleteDirectory(r.URL.Path, isRecursive)
 	} else {
 		fid, err = fs.filer.DeleteFile(r.URL.Path)
-		if err == nil {
+		if err == nil && fid != "" {
 			err = operation.DeleteFile(fs.master, fid, fs.jwt(fid))
 		}
 	}