6LoWPAN: fix storage of contexts coming from ICMPv6 Router Advertisement messages

Bug: 10443
Change-Id: I1a74b4d3036c61f0c8e1fca6373845ea40d87808
Reviewed-on: https://code.wireshark.org/review/4008
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>