RETURN micro optimization

Hi, I propose a micro optimization for RETURN statement. Currently "return $x" increments reference counter of $x, then in zend_leave_helper() we perform zval_ptr_dtor() on the same $x. The patch sets the original value of $x to null in first place, so zval_ptr_dtor() is not going to be called. https://gist.github.com/dstogov/36f68b206242a39691ac539c2fc85d40 the performance impact is invisible (0.1% less instruction retired on Wordpress). It breaks sapi/phpdbg/tests/breakpoints_005.phpt, but this is probably not a big deal. BTW: this change may affect debuggers in some other way. Let me know, if you see any problems (I'll delay commit for 2-3 days). Thanks. Dmitry.