sctp: improve handling of illegal packets containing INIT chunks

Stop further processing of a packet when detecting that it
contains an INIT chunk, which is too small or is not the only
chunk in the packet. Still allow to finish the processing
of chunks before the INIT chunk.

Thanks to Antoly Korniltsev and Taylor Brandstetter for reporting
an issue with the userland stack, which made me aware of this
issue.

(cherry picked from commit c70d1ef15db0d994eff4a2c4d9feabdc46bff1c6)