The NetstackDialTCP/UDP hooks returned the result of DialContextTCP/UDP
directly, so on error they returned a non-nil net.Conn interface holding
a nil *gonet.TCPConn or *gonet.UDPConn pointer, tripping up callers that
check the interface against nil and then call Close, crashing the wasm
worker. Apply the same fix that 46bdbb387 made for tailscaled and tsnet.
Fixes#20529
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I4fd66bb7615ee9b2d204256a43288ed7b7a12f35