client: don't call callbacks if close() is called (#258)

- ts-protoc-gen clears out the callbacks anyways
- Fixes #257

ts/src/client.ts

@@ -209,6 +209,7 @@ class GrpcClient<TRequest extends ProtobufMessage, TResponse extends ProtobufMes
 
     this.onEndCallbacks.forEach(callback => {
       detach(() => {
+        if (this.closed) return;
         callback(code, message, trailers);
       });
     });
@@ -230,6 +231,7 @@ class GrpcClient<TRequest extends ProtobufMessage, TResponse extends ProtobufMes
     this.completed = true;
     this.onEndCallbacks.forEach(callback => {
       detach(() => {
+        if (this.closed) return;
         callback(code, msg, trailers);
       });
     });
@@ -240,6 +242,7 @@ class GrpcClient<TRequest extends ProtobufMessage, TResponse extends ProtobufMes
     if (this.completed || this.closed) return;
     this.onMessageCallbacks.forEach(callback => {
       detach(() => {
+        if (this.closed) return;
         callback(res);
       });
     });