api/handler/rpc: fix panic on invalid error conversation (#1483)

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

api/handler/rpc/stream.go

@@ -183,17 +183,17 @@ func writeLoop(rw io.ReadWriter, stream client.Stream) {
 		default:
 			buf, op, err := wsutil.ReadClientData(rw)
 			if err != nil {
-				wserr := err.(wsutil.ClosedError)
+				if wserr, ok := err.(wsutil.ClosedError); ok {
 					switch wserr.Code {
 					case ws.StatusNormalClosure, ws.StatusNoStatusRcvd:
 						return
-				default:
+					}
+				}
 				if logger.V(logger.ErrorLevel, logger.DefaultLogger) {
 					logger.Error(err)
 				}
 				return
 			}
-			}
 			switch op {
 			default:
 				// not relevant