net/socket.c: Fix fd leak in net_socket_listen_init() error paths

Fix a leak of a file descriptor due to missing closesocket() calls
in error paths in net_socket_listen_init().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
