Don't check the checksum if we're encapsulated in an (ICMP) error packet.

This is the case when an ICMP echo is returned (e.g., because it's too big).

This effectively applies SVN rev 36193 to ICMPv6.

Bug: 10620
Change-Id: I29aeaabc2467c818714df448e39b3a93027c8961
Reviewed-on: https://code.wireshark.org/review/5030
Petri-Dish: Jeff Morriss <jeff.morriss.ws@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>