Fix issue #624

A cancelled future in the flush_queue can cause the flusher task to fail with InvalidStateError, attempting to set the result on a "done" future.

nats/aio/client.py

@@ -2084,7 +2084,9 @@ async def _flusher(self) -> None:
                 # RuntimeError in case the event loop is closed
                 break
             finally:
-                future.set_result(None)
+                # future might have been cancelled.  See issue #624
+                if not future.done():
+                    future.set_result(None)
 
     async def _ping_interval(self) -> None:
         while True: