Fix self-assignment in folly::coro::AsyncPipe

Reviewed By: yfeldblum

Differential Revision: D22843001

fbshipit-source-id: 55f48de90f16c53983ea94c8e1102c5b33e25195

folly/experimental/coro/AsyncPipe.h

@@ -48,8 +48,10 @@ class AsyncPipe {
     queue_ = std::move(pipe.queue_);
   }
   AsyncPipe& operator=(AsyncPipe&& pipe) {
+    if (this != &pipe) {
       std::move(*this).close();
       queue_ = std::move(pipe.queue_);
+    }
     return *this;
   }