MIPv6: fix dissection of Service Selection Identifier

APN field is not encoded as a dotted string so the first character is not a length

Bug: 10323
Change-Id: Ia62137c785d505e9d0f1536a333b421a85480741
Reviewed-on: https://code.wireshark.org/review/3461
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>