[unix(4) cleanup 8/8] Fix a double connect race.

[markj]

Summary

dgram sockets may be connected multiple times. If connect() is called
on a connected dgram socket, soconnectat() calls sodisconnect() before
creating the new connection. However, this is not synchronized, so two
concurrent connect() calls can race with each other, and uipc_connect()
does not handle it. Add an explicit check for unp_conn != NULL in
unp_connectat().

Reported by: syzkaller

[markj]

Subsumed by D26299.