Fix panic on TCP packets of an unconfigured address family in system stack (#22)

(cherry picked from commit f3b42b8)

---------

Co-authored-by: 世界 <i@sekai.icu>

Author: chenx-dust

stack_system.go

@@ -401,6 +401,9 @@ func (s *System) processIPv6(ipHdr header.IPv6) (writeBack bool, err error) {
 }
 
 func (s *System) processIPv4TCP(ipHdr header.IPv4, tcpHdr header.TCP) (bool, error) {
+	if s.tcpNat4 == nil {
+		return false, nil
+	}
 	source := netip.AddrPortFrom(ipHdr.SourceAddr(), tcpHdr.SourcePort())
 	destination := netip.AddrPortFrom(ipHdr.DestinationAddr(), tcpHdr.DestinationPort())
 	if !destination.Addr().IsGlobalUnicast() {
@@ -492,6 +495,9 @@ func (s *System) resetIPv4TCP(origIPHdr header.IPv4, origTCPHdr header.TCP) erro
 }
 
 func (s *System) processIPv6TCP(ipHdr header.IPv6, tcpHdr header.TCP) (bool, error) {
+	if s.tcpNat6 == nil {
+		return false, nil
+	}
 	source := netip.AddrPortFrom(ipHdr.SourceAddr(), tcpHdr.SourcePort())
 	destination := netip.AddrPortFrom(ipHdr.DestinationAddr(), tcpHdr.DestinationPort())
 	if !destination.Addr().IsGlobalUnicast() {