GHSA-xjgr-2wmf-f78j: NULL pointer dereference in ICMPv6 Packet Too Big processing when the destination table is full

In Eclipse ThreadX NetX Duo versions 6.4.5.202504 through 6.5.2.202603a, built with IPv6 path MTU discovery, the ICMPv6 Packet Too Big handler uses the destination-table entry it adds without checking that the add succeeded. A sender on the network can fill the destination table with Packet Too Big messages quoting new destinations, after which the next message makes the IP helper thread dereference a NULL pointer, crashing it on targets that fault on address 0 and stopping all network processing.