Revert "Share the message buffer if we have room"

This reverts commit 17ce0cd52f.

msg.go

@@ -254,15 +254,7 @@ func packDomainName(s string, msg []byte, off int, compression map[string]int, c
 	pointer := -1
 	// Emit sequence of counted strings, chopping at dots.
 	begin := 0
-	// We can share the message buffer because we don't need to look back.
-	// Keeping close makes tests more sensitive to failure (only 1 byte of room).
-	var bs []byte
-	if len(s) < len(msg) {
-		bs = msg[off+1:]
-		copy(bs, s)
-	} else {
-		bs = []byte(s)
-	}
+	bs := []byte(s)
 	ro_bs, bs_fresh := s, true
 	for i := 0; i < ls; i++ {
 		if bs[i] == '\\' {