Fix possible race in isc__nm_tcpconnect.

Fix possible race in isc__nm_tcpconnect.

There's a possibility of race in isc__nm_tcpconnect if the asynchronous
connect operation finishes with all the callbacks before we exit the
isc__nm_tcpconnect itself we might access an already freed memory.
Fix it by creating an additional reference to the socket freed at the
end of isc__nm_tcpconnect.

(cherry picked from commit 896db0f41988f66bd06d6fa840b18be66a22200a)