in open_table()

Problem: repeating "CREATE... ( AUTOINCREMENT) ... SELECT" may lead to
an assertion failure.

Fix: reset table->auto_increment_field_not_null after each record 
writing.


mysql-test/r/create.result:
  Fix for bug#38821: Assert table->auto_increment_field_not_null failed 
  in open_table()
    - test result.
mysql-test/t/create.test:
  Fix for bug#38821: Assert table->auto_increment_field_not_null failed 
  in open_table()
    - test case.
sql/sql_insert.cc:
  Fix for bug#38821: Assert table->auto_increment_field_not_null failed 
  in open_table()
    - reset table->auto_increment_field_not_null after writing a record
  for "{CREATE, INSERT}..SELECT".