Websocket disconnected every 300 sec on windows because invalid errno handling

[andrewthecat]

errno set here first:

https://github.com/eclipse-mosquitto/mosquitto/blob/develop/lib/net_ws.c#L304

but overwritten only on windows here:

https://github.com/eclipse-mosquitto/mosquitto/blob/develop/lib/packet_mosq.c#L367

this breaks ws ping logic