zend_compile: Fix zend_compile_rope_finalize() for ropes containing a single constant string

Without this Opcache will trigger a use-after-free in
`zend_optimizer_compact_literals()`.

Co-authored-by: Ilija Tovilo <ilija.tovilo@me.com>

Author: TimWolla

Zend/zend_compile.c

@@ -10193,6 +10193,8 @@ static void zend_compile_rope_finalize(znode *result, uint32_t rope_elements, ze
 	if (rope_elements == 1) {
 		if (opline->op2_type == IS_CONST) {
 			GET_NODE(result, opline->op2);
+			ZVAL_UNDEF(CT_CONSTANT(opline->op2));
+			SET_UNUSED(opline->op2);
 			MAKE_NOP(opline);
 		} else {
 			opline->opcode = ZEND_CAST;