6LoWPAN: use manually managed memory for the stateful address decompression preferences

Bug: 10747
Change-Id: I9d1b7ce2782f7cd133ab3ba02b0dd43afab3e145
Reviewed-on: https://code.wireshark.org/review/5560
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>