Fix message_update_7013 failing on PostgreSQL.

[RoSk0]

Latest message update message_update_7013() is failing on PostgreSQL with :

SQLSTATE[42P07]: Duplicate table: 7 ERROR: relation "message_mid_seq" already exists.

because on PostgreSQL serial fields are not NULLs by default. Check Drupal driver for PostgreSQL here includes/database/pgsql/schema.inc:181 DatabaseSchema_pgsql::protected function createFieldSql()

if (isset($spec['type']) && $spec['type'] == 'serial') {
  unset($spec['not null']);
}

and includes/database/pgsql/schema.inc:252 DatabaseSchema_pgsql::protected function processField()

if (isset($field['type']) && $field['type'] == 'serial') {
      unset($field['not null']);
}