Bug #66481 Segfaults on session_name()

If the previous value of session.name was NULL then any call to
session_name($string) would result in a segmentation fault.

This changes the behaviour to set the value of session.name to
"PHPSESSID" if a blank value is given in php.ini or via -d on the
command line. There is already protection against setting it to NULL via
session_name() or ini_set().