Downloader: Join threads in CancelAll

ext/native/net/http_client.cpp

@@ -446,6 +446,9 @@ void Download::Start() {
 }
 
 void Download::Join() {
+	if (joined_) {
+		ELOG("Already joined thread!");
+	}
 	thread_.join();
 	joined_ = true;
 }
@@ -585,6 +588,9 @@ void Downloader::CancelAll() {
 	for (size_t i = 0; i < downloads_.size(); i++) {
 		downloads_[i]->Cancel();
 	}
+	for (size_t i = 0; i < downloads_.size(); i++) {
+		downloads_[i]->Join();
+	}
 	downloads_.clear();
 }