Fix copy() in SVCBIPv4Hint and SVCBIPv6Hint (#1256)

* Fix copy() in SVCBIPv4Hint and SVCBIPv6Hint

The problem with the current implementation is that it is not a real deep copy,
it points to the same base arrays behind the slices. This was causing some
issues in real-life application.

* Address review comments

svcb.go

@@ -511,8 +511,13 @@ func (s *SVCBIPv4Hint) parse(b string) error {
 }
 
 func (s *SVCBIPv4Hint) copy() SVCBKeyValue {
+	hint := make([]net.IP, len(s.Hint))
+	for i, ip := range s.Hint {
+		hint[i] = copyIP(ip)
+	}
+
 	return &SVCBIPv4Hint{
-		append([]net.IP(nil), s.Hint...),
+		Hint: hint,
 	}
 }
 
@@ -629,8 +634,13 @@ func (s *SVCBIPv6Hint) parse(b string) error {
 }
 
 func (s *SVCBIPv6Hint) copy() SVCBKeyValue {
+	hint := make([]net.IP, len(s.Hint))
+	for i, ip := range s.Hint {
+		hint[i] = copyIP(ip)
+	}
+
 	return &SVCBIPv6Hint{
-		append([]net.IP(nil), s.Hint...),
+		Hint: hint,
 	}
 }