dns/dnsmessage: avoid string(i) where i has type int
dns/dnsmessage: avoid string(i) where i has type int

Instead use string(r) where r has type rune.

This is in preparation for a vet warning for string(i).

Updates golang/go#32479

Change-Id: I778323fe01eeac4a1068a350c965039eef84df30
Reviewed-on: https://go-review.googlesource.com/c/net/+/220798
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Akhil Indurti <28f6676df8cca80b7030f6241b450cfee74a2924@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Toshihiro Shiino <23294d23219951288b28e9546aefaf9eca8d87c6@gmail.com>

golang/net