SCTP: Fix chunk acceptance when no authenticated chunks were listed.

In the case where no autheticated chunks were specified, we were still
trying to verify that a given chunk needs authentication and doing so
incorrectly.  Add a check for parameter length to make sure we don't
try to use an empty auth_chunks parameter to verify against.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>