[Migration] Fixes issue when user_id already exists with the api table.

application/migrations/079_add_userid_to_api.php

@@ -9,8 +9,10 @@ class Migration_add_userid_to_api extends CI_Migration
 			'user_id BIGINT(20) DEFAULT NULL',
 		);
 
+		if (!$this->db->field_exists('user_id', 'api')) {
 			$this->dbforge->add_column('api', $fields);
 		}
+	}
 
 	public function down()
 	{