A collection of small fixes for the SCRAM patch.

* Add required #includes for htonl. Per buildfarm members pademelon/gaur.

* Remove unnecessary "#include <utils/memutils>".

* Fix checking for empty string in pg_SASL_init. (Reported by Peter
  Eisentraut and his compiler)

* Move code in pg_SASL_init to match the recent changes (commit ba005f193d)
  to pg_fe_sendauth() function, where it's copied from.

* Return value of malloc() was not checked for NULL in
  scram_SaltedPassword(). Fix by avoiding the malloc().
