fix crash on sendKeepalive fail

Summary:
if the socket connection is closed (or a close is detected) remotely during sendKeepalive then
* stop() gets invoked and
* the connection_ pointer is cleared and
* pending_ is set to false
* but pending_ will still be set to true after this returns, and you'll get a crash as the connection_ shared_ptr is now null.

swapping the order allows stop() to clear the pending_ flag properly if send fails.

Reviewed By: phoad

Differential Revision: D8926438

fbshipit-source-id: 897a46abc70a50d1e6cce50c0502a4cd86d6d4ae

Author: Aaron Brady

rsocket/internal/KeepaliveTimer.cpp

@@ -53,8 +53,10 @@ void KeepaliveTimer::sendKeepalive() {
     localPtr->disconnectOrCloseWithError(
         Frame_ERROR::connectionError("no response to keepalive"));
   } else {
-    connection_->sendKeepalive();
+    // this must happen before sendKeepalive as it can potentially result in
+    // stop() being called
     pending_ = true;
+    connection_->sendKeepalive();
     schedule();
   }
 }