Assertion `!pk->has_virtual()' failed in dict_index_build_internal_clust
while creating PRIMARY key longer than possible to store in the page.

This happened because the key was wrongly deduced as Long UNIQUE supported,
however PRIMARY KEY cannot be of that type. The main reason is that
only 8 bytes are used to store the hash, see HA_HASH_FIELD_LENGTH.

This is also why HA_NOSAME flag is removed (and caused the assertion in
turn) in open_table_from_share:
      if (key_info->algorithm == HA_KEY_ALG_LONG_HASH)
      {
        key_part_end++;
        key_info->flags&= ~HA_NOSAME;
      }

To make it unique, the additional check is done by
check_duplicate_long_entries call from ha_write_row, and similar one from
ha_update_row.

PRIMARY key is already forbidden, which is checked by the first test in
main.long_unique, however is_hash_field_needed was wrongly deduced to true
in mysql_prepare_create_table in this particular case.

FIX:

* Improve the check for Key::PRIMARY type
* Simplify is_hash_field_needed deduction for a more neat reading