DNS: fix dissection of ISDN RR Sub Address field

Bug: 10650
Change-Id: I3411d4e5ef534a663940974d98c351976005ee8e
Reviewed-on: https://code.wireshark.org/review/5057
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>