MINOR: htx: switch from conn->addr.{from,to} to conn->{src,dst}

MINOR: htx: switch from conn->addr.{from,to} to conn->{src,dst}

One place (transparent proxy) will require an allocation when the
address becomes dynamic. A few dereferences of the family were adjusted
to preliminary check for the address pointer to exist at all. The
remaining operations were already performed under control of a
successful retrieval.