When using KTRACE, set a variable to the appropriate value and don't
leave it initialized at NULL.
Since the affected functions where moved from sys/kern/uipc_syscalls.c
to sys/netinet/sctp_syscalls.c it was not possible to MFC r284613.
Therefore, this is a direct commit with the corresponding changes of r284613.

Reported by: Coverity
CID: 1018058, 1018060