ospf6d: make some old sizing checks assertions

ospf6d: make some old sizing checks assertions

As long as ospf6_packet_examin() is now the single checkpoint for
received packets, most of the old checks performed elsewhere can
be converted into assert() constructs. Malformed input data at
respective points can be attributed solely to a programming error,
not a malformed packet.

* ospf6_message.c
  * ospf6_hello_print()
  * ospf6_dbdesc_print()
  * ospf6_lsreq_print()
  * ospf6_lsupdate_print()
  * ospf6_lsack_print()
  * ospf6_hello_recv()
  * ospf6_dbdesc_recv_master()
  * ospf6_dbdesc_recv_slave()
  * ospf6_lsreq_recv()
  * ospf6_lsupdate_recv()
  * ospf6_lsupdate_recv()
  * ospf6_lsack_recv()
  * ospf6_receive()