Colin's commit went a step further and protected tun.incomingPacket with
a lock on shutdown, but let's see if the tun.stack.Close() call actually
solves that on its own.
Suggested-by: kshangx <hikeshang@hotmail.com>
Suggested-by: Colin Adler <colin1adler@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>