utp_socket memory can be leaked

[anacrolix]

If utp_create_socket is called, and then utp_connect is not called, there's no way to reclaim the utp_socket memory. If the parent utp_context is destroyed, the memory for the unconnected utp_socket is not reclaimed, and it's no longer possible to call utp_connect.

sock = utp_create_socket(ctx)
utp_destroy(ctx)
# segfaults, no way to reclaim sock memory
utp_connect(sock)

Here's another sequence:

sock = utp_create_socket(ctx)
utp_destroy(ctx)
# asserts
utp_close(sock)

I think utp_connect should not segfault if the parent utp_context is destroyed, and utp_close should probably delete the utp_socket if it's uninitialized.