REDHAT-BUG-2418832: Medium severity linux/kernel vulnerability

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