QUIC: fixed post-close use-after-free. Previously, ngx_quic_close_connection() could be called in a way that QUIC connection was accessed after the call. In most cases the connection is not closed right away, but close timeout is scheduled. However, it's not always the case. Also, if the close process started earlier for a different reason, calling ngx_quic_close_connection() may actually close the connection. The connection object should not be accessed after that. Now, when possible, return statement is added to eliminate post-close connection object access. In other places ngx_quic_close_connection() is substituted with posting close event. Also, the new way of closing connection in ngx_quic_stream_cleanup_handler() fixes another problem in this function. Previously it passed stream connection instead of QUIC connection to ngx_quic_close_connection(). This could result in incomplete connection shutdown. One consequence of that could be that QUIC streams were freed without shutting down their application contexts. This could result in another use-after-free. Found by Coverity (CID 1530402).