accept-phase: shutdown socket on early error

accept-phase: shutdown socket on early error

if an error happens before AnyEvent::Handle registers the cleanup
callback, we should shutdown/close the socket, when handling it.

Using close, instead of shutdown($sock, SHUT_WR) here, since we are in
an error-state, and would not read from the socket anyways.
(Additionally close sends just on packet (RST,ACK), vs shutdown
(FIN,ACK+RST,ACK) in its use here).

Co-Authored-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>