cmd/cloner,*: revert: optimize nillable slice cloner

This reverts commit ee90cd02fd.

The outcome is not identical for empty slices. Cloner really needs
tests!

Updates #9601

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/filter/filter_clone.go

@@ -23,9 +23,11 @@ func (src *Match) Clone() *Match {
 	dst.IPProto = append(src.IPProto[:0:0], src.IPProto...)
 	dst.Srcs = append(src.Srcs[:0:0], src.Srcs...)
 	dst.Dsts = append(src.Dsts[:0:0], src.Dsts...)
-	dst.Caps = append([]CapMatch(nil), make([]CapMatch, len(src.Caps))...)
-	for i := range dst.Caps {
-		dst.Caps[i] = *src.Caps[i].Clone()
+	if src.Caps != nil {
+		dst.Caps = make([]CapMatch, len(src.Caps))
+		for i := range dst.Caps {
+			dst.Caps[i] = *src.Caps[i].Clone()
+		}
 	}
 	return dst
 }

wgengine/wgcfg/wgcfg_clone.go

@@ -24,9 +24,11 @@ func (src *Config) Clone() *Config {
 	*dst = *src
 	dst.Addresses = append(src.Addresses[:0:0], src.Addresses...)
 	dst.DNS = append(src.DNS[:0:0], src.DNS...)
-	dst.Peers = append([]Peer(nil), make([]Peer, len(src.Peers))...)
-	for i := range dst.Peers {
-		dst.Peers[i] = *src.Peers[i].Clone()
+	if src.Peers != nil {
+		dst.Peers = make([]Peer, len(src.Peers))
+		for i := range dst.Peers {
+			dst.Peers[i] = *src.Peers[i].Clone()
+		}
 	}
 	return dst
 }