INSERT/UPDATE against CSV table created by MySQL earlier than 5.1.23
with NULL-able column results in server crash in debug builds.

Starting with 5.1.23 CSV doesn't permit creation of NULL-able columns,
but it is still possible to get such table from older MySQL versions.

Fixed by removing excessive DBUG_ASSERT().


scripts/mysql_system_tables_fix.sql:
  Starting with 5.1.23 CSV doesn't permit creation of NULL-able columns.
  Alter system CSV tables structure so that all columns are NOT NULL.
storage/csv/ha_tina.cc:
  Starting with 5.1.23 CSV doesn't permit creation of NULL-able columns,
  but it is still possible to get such table from older MySQL versions.
  
  Removed excessive DBUG_ASSERT().