Fix leak with nullsafe operator with constant LHS

Followup to 5303bcd. We need to
perform the addref after emitting the opline, because that might
intern the string.

Fixes oss-fuzz #24479.

Author: nikic

Zend/tests/nullsafe_operator/029.phpt

@@ -3,6 +3,9 @@ Refcount of constant LHS with nullsafe operator
 --FILE--
 <?php
 ['']?->a;
+__DIR__?->a;
 ?>
 --EXPECTF--
 Warning: Attempt to read property "a" on array in %s on line %d
+
+Warning: Attempt to read property "a" on string in %s on line %d

Zend/zend_compile.c

@@ -2308,10 +2308,10 @@ static void zend_short_circuiting_commit(uint32_t checkpoint, znode *result, zen
 static void zend_emit_jmp_null(znode *obj_node)
 {
 	uint32_t jmp_null_opnum = get_next_op_number();
-	if (obj_node->op_type == IS_CONST) {
-		Z_TRY_ADDREF(obj_node->u.constant);
+	zend_op *opline = zend_emit_op(NULL, ZEND_JMP_NULL, obj_node, NULL);
+	if (opline->op1_type == IS_CONST) {
+		Z_TRY_ADDREF_P(CT_CONSTANT(opline->op1));
 	}
-	zend_emit_op(NULL, ZEND_JMP_NULL, obj_node, NULL);
 	zend_stack_push(&CG(short_circuiting_opnums), &jmp_null_opnum);
 }