CVE-2025-40240

In the Linux kernel, the following vulnerability has been resolved:

sctp: avoid NULL dereference when chunk data buffer is missing

chunk->skb pointer is dereferenced in the if-block where it's supposed
to be NULL only.

chunk->skb can only be NULL if chunk->head_skb is not. Check for frag_list
instead and do it just before replacing chunk->skb. We're sure that
otherwise chunk->skb is non-NULL because of outer if() condition.