mount: release need to avoid nil reader

fix https://github.com/chrislusf/seaweedfs/issues/1803

weed/filesys/filehandle.go

@@ -196,8 +196,10 @@ func (fh *FileHandle) Release(ctx context.Context, req *fuse.ReleaseRequest) err
 
 		fh.f.wfs.ReleaseHandle(fh.f.fullpath(), fuse.HandleID(fh.handle))
 		if closer, ok := fh.f.reader.(io.Closer); ok {
+			if closer != nil {
 				closer.Close()
 			}
+		}
 		fh.f.reader = nil
 	}