Fix socket leak

After running for a while (and thrashing connections doing a live-update thing), connections began to fail like this:

[debug] accept() failed: 24
[debug] awake on incoming
[debug] accept() failed: 24
[debug] awake on incoming

$ lsof -p 10680
daedalus 10680 paulholden    8u    IPv4 0x61822fb722019d87        0t0      TCP localhost:sunproxyadmin (LISTEN)
daedalus 10680 paulholden    9u    IPv4 0x61822fb720940c8f        0t0      TCP localhost:sunproxyadmin->localhost:62430 (CLOSE_WAIT)
daedalus 10680 paulholden   10u    IPv4 0x61822fb7208d3c8f        0t0      TCP localhost:sunproxyadmin->localhost:62431 (CLOSE_WAIT)
daedalus 10680 paulholden   11u    IPv4 0x61822fb71aa657df        0t0      TCP localhost:sunproxyadmin->localhost:62432 (CLOSE_WAIT)
daedalus 10680 paulholden   12u    IPv4 0x61822fb72200daff        0t0      TCP localhost:sunproxyadmin->localhost:62433 (CLOSE_WAIT)
...
daedalus 10680 paulholden  252u    IPv4 0x61822fb71997364f        0t0      TCP localhost:sunproxyadmin->localhost:62675 (CLOSE_WAIT)
daedalus 10680 paulholden  253u    IPv4 0x61822fb71997196f        0t0      TCP localhost:sunproxyadmin->localhost:62676 (CLOSE_WAIT)
daedalus 10680 paulholden  254u    IPv4 0x61822fb71996fc8f        0t0      TCP localhost:sunproxyadmin->localhost:62677 (CLOSE_WAIT)
daedalus 10680 paulholden  255u    IPv4 0x61822fb71998dd87        0t0      TCP localhost:sunproxyadmin->localhost:62678 (CLOSE_WAIT)

My fix just checks connection->socket is still valid rather than the WB_ALIVE flag, which fixes the issue above.

(WebbyServerShutdown tears down sockets without doing any checking - maybe that should check socket != WB_INVALID_SOCKET too?)