Description:
------------
in mod_use_class.c

there is a macro:
#define PS_SANITY_CHECK                     \
    if (PS(default_mod) == NULL) {              \
        php_error_docref(NULL TSRMLS_CC, E_CORE_ERROR, "Called default 
SessionHandler but session.save_handler is user"); \
        RETURN_FALSE;                       \
    }    

but I think this is wrong. 

since, the default session.save_handler is "file", and if no new 
session.save_handler provide, PS(default_mod) will alyways be NULL(since 
default_mod only be assign in OnUpdateSaveHandler

Test script:
---------------
php -dsession.save_handler=files  -r 'var_dump(ini_get("session.save_handler")); $x=new SessionHandler; $x->gc(1);' 

Actual result:
--------------
PHP Fatal error:  SessionHandler::gc(): Called default SessionHandler but 
session.save_handler is user in Unknown on line 0