avoid overwriting non-nil uploadReaderToChunks.uploadErr with nil value (#4560)

weed/server/filer_server_handlers_write_upload.go

@@ -70,9 +70,13 @@ func (fs *FilerServer) uploadReaderToChunks(w http.ResponseWriter, r *http.Reque
 		if err != nil || dataSize == 0 {
 			bufPool.Put(bytesBuffer)
 			<-bytesBufferLimitChan
+			if err != nil {
 				uploadErrLock.Lock()
+				if uploadErr == nil {
 					uploadErr = err
+				}
 				uploadErrLock.Unlock()
+			}
 			break
 		}
 		if chunkOffset == 0 && !isAppend {