fix: prevent panic on network change during SetSelfIP

SetSelfIP unconditionally accessed t.dhcp, but t.dhcp is only
initialized when DHCP discovery is enabled. A network change event
can fire SetSelfIP regardless of the discovery configuration,
causing a nil pointer dereference.

Guard the t.dhcp access with a nil check so the self IP is still
updated on the Table even when DHCP discovery is disabled.

internal/clientinfo/client_info.go

@@ -173,8 +173,10 @@ func (t *Table) SetSelfIP(ip string) {
 	t.selfIPLock.Lock()
 	defer t.selfIPLock.Unlock()
 	t.selfIP = ip
-	t.dhcp.selfIP = t.selfIP
-	t.dhcp.addSelf()
+	if t.dhcp != nil {
+		t.dhcp.selfIP = t.selfIP
+		t.dhcp.addSelf()
+	}
 }
 
 // initSelfDiscover initializes necessary client metadata for self query.