SSCursor: Fix connection closed check

Since the connection closes itself on an `asyncio.CancelledError`, it
cannot be used afterwards. `SSCursor.close` attempts to clear some
unfinished state first before closing using
`Connection._finish_unbuffered_query`, but that call fails if the
connection has been closed already and SSCursor itself does not
properly check that.

aiomysql/cursors.py

@@ -598,7 +598,8 @@ class SSCursor(Cursor):
 
     async def close(self):
         conn = self._connection
-        if conn is None:
+        if conn is None or conn.closed:
+            self._connection = None
             return
 
         if self._result is not None and self._result is conn._result: