Fixed IS_INDIRECT handling when assign array element by reference.

Fixed IS_INDIRECT handling when assign array element by reference.
This might cause passing invalid pointer if the array could be resized before IS_INDIRECT usage.
Now we use IS_REFERENCE instead of IS_INDIRECT.
The following test crashed before:
<?php $a = array(0,1,2,3,4,5,6,7); $a[8] = &$a[7]; ?>