net/portmapper: don't treat 0.0.0.0 as a valid IP

Updates tailscale/corp#23538

Change-Id: I58b8c30abe43f1d1829f01eb9fb2c1e6e8db9476
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/portmapper/portmapper.go

@@ -781,6 +781,10 @@ func parsePMPResponse(pkt []byte) (res pmpResponse, ok bool) {
 			return res, false
 		}
 		res.PublicAddr = netaddr.IPv4(pkt[8], pkt[9], pkt[10], pkt[11])
+		if res.PublicAddr.IsUnspecified() {
+			// Zero it out so it's not Valid and used accidentally elsewhere.
+			res.PublicAddr = netip.Addr{}
+		}
 	}
 
 	return res, true