Avoid error on process wait() followed by wait_closed()

This commit prevents a RuntimeException from occurring when a process
has wait() called on it and later a call is made to wait_closed() (such
as from a context manager). Thanks go to GitHub user starflows for
reporting this issue!

asyncssh/process.py

@@ -1210,6 +1210,8 @@ async def wait_closed(self) -> None:
         for task in self._cleanup_tasks:
             await task
 
+        self._cleanup_tasks = []
+
 
 class SSHClientProcess(SSHProcess[AnyStr], SSHClientStreamSession[AnyStr]):
     """SSH client process handler"""