In tcp_reass(), assert the inpcb lock on the passed tcpcb, since the
contents of the tcpcb are read and modified in volume.

In tcp_input(), replace th comparison with 0 with a comparison with
NULL.

At the 'findpcb', 'dropafterack', and 'dropwithreset' labels in
tcp_input(), assert 'headlocked'. Try to improve consistency between
various assertions regarding headlocked to be more informative.

MFC after: 2 weeks