ipn/ipnlocal: allow Peer API access via either V4MasqAddr or V6MasqAddr when both are set

This doesn't seem to have any immediate impact, but not allowing access via the IPv6 masquerade
address when an IPv4 masquerade address is also set seems like a bug.

Updates #cleanup
Updates #14570 (found when working on it)

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/peerapi.go

@@ -233,11 +233,13 @@ func (h *peerAPIHandler) logf(format string, a ...any) {
 // isAddressValid reports whether addr is a valid destination address for this
 // node originating from the peer.
 func (h *peerAPIHandler) isAddressValid(addr netip.Addr) bool {
-	if v, ok := h.peerNode.SelfNodeV4MasqAddrForThisPeer().GetOk(); ok {
-		return v == addr
+	if !addr.IsValid() {
+		return false
 	}
-	if v, ok := h.peerNode.SelfNodeV6MasqAddrForThisPeer().GetOk(); ok {
-		return v == addr
+	v4MasqAddr, hasMasqV4 := h.peerNode.SelfNodeV4MasqAddrForThisPeer().GetOk()
+	v6MasqAddr, hasMasqV6 := h.peerNode.SelfNodeV6MasqAddrForThisPeer().GetOk()
+	if hasMasqV4 || hasMasqV6 {
+		return addr == v4MasqAddr || addr == v6MasqAddr
 	}
 	pfx := netip.PrefixFrom(addr, addr.BitLen())
 	return views.SliceContains(h.selfNode.Addresses(), pfx)