Destroy the awaitable's frame as the result is consumed.

This change works around a gcc 10 bug, where the temporary awaitable
object being awaited is not destroyed if the co_await resumes with an
exception.

asio/include/asio/awaitable.hpp

@@ -100,7 +100,7 @@ public:
   // Support for co_await keyword.
   T await_resume()
   {
-    return frame_->get();
+    return awaitable(static_cast<awaitable&&>(*this)).frame_->get();
   }
 
 #endif // !defined(GENERATING_DOCUMENTATION)