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

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>

net/netcheck/netcheck.go

@@ -40,6 +40,7 @@
 	"tailscale.com/types/logger"
 	"tailscale.com/types/nettype"
 	"tailscale.com/types/opt"
+	"tailscale.com/types/ptr"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/mak"
 )
@@ -943,6 +944,16 @@ func (c *Client) GetReport(ctx context.Context, dm *tailcfg.DERPMap) (_ *Report,
 				go c.readPackets(ctx, u6)
 			}
 		}
+
+		// If our interfaces.State suggested we have IPv6 support but then we
+		// failed to get an IPv6 sending socket (as in
+		// https://github.com/tailscale/tailscale/issues/7949), then change
+		// ifState.HaveV6 before we make a probe plan that involves sending IPv6
+		// packets and thus assuming rs.pc6 is non-nil.
+		if rs.pc6 == nil {
+			ifState = ptr.To(*ifState) // shallow clone
+			ifState.HaveV6 = false
+		}
 	}
 
 	plan := makeProbePlan(dm, ifState, last)