server: Hold socket reference in sock_poll_event().

Open Paul Gofman requested to merge gofman/wine:sock_poll_event_ref into master

The socket may get released if the sock_poll_event() is inspired by closing socket.

Here is a WIP test (where I am trying to test unrelated things) which causes assert in wineserver on closing socket (when server is trying to use already freed socket): https://gist.github.com/gofman/cc78a07ed58f6d02ff863388d5992acb

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