Avoid dereferencing a nullptr socket_

Summary: Avoid dereferencing a nullptr socket_

Reviewed By: danobi

Differential Revision: D24652875

fbshipit-source-id: 63a9535226c8ff58fd5348f3c38f5c6c498d9527

folly/io/async/test/AsyncUDPSocketTest.cpp

@@ -208,9 +208,11 @@ class UDPClient : private AsyncUDPSocket::ReadCallback, private AsyncTimeout {
 
   void shutdown() {
     CHECK(evb_->isInEventBaseThread());
-    socket_->pauseRead();
-    socket_->close();
-    socket_.reset();
+    if (socket_) {
+      socket_->pauseRead();
+      socket_->close();
+      socket_.reset();
+    }
     evb_->terminateLoopSoon();
   }