dccp: Add check for truncated ICMPv6 DCCP error packets

This patch adds a minimum-length check for ICMPv6 packets, as per the previous
patch for ICMPv4 payloads.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>