net/netcheck: fix crash when IPv6 kinda but not really works

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Looks like on some systems there's an IPv6 address, but then opening
a IPv6 UDP socket fails later. Probably some firewall. Tolerate it
better and don't crash.

To repro: check the "udp6" to something like "udp7" (something that'll
fail) and run "go run ./cmd/tailscale netcheck" on a machine with
active IPv6. It used to crash and now it doesn't.

Fixes #7949

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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