Partial fix that allows internal constructors to set $this to null.
The address of $this passed to drectly called internal constructor in execute_data->return_value.
Internal constructors should use ZEND_CTOR_MAKE_NULL() macro (insted of previous ZEND_NULL(EG(This))) to do the work.

This patch doesn't fix the problem for indirectly called constructors. e.g. parant::__construct().