Fix use-after-free when nullsafe used with constant LHS

Fixes oss-fuzz #24436.

Author: nikic

Zend/tests/nullsafe_operator/029.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Refcount of constant LHS with nullsafe operator
+--FILE--
+<?php
+['']?->a;
+?>
+--EXPECTF--
+Warning: Attempt to read property "a" on array in %s on line %d

Zend/zend_compile.c

@@ -2308,6 +2308,9 @@ 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_emit_op(NULL, ZEND_JMP_NULL, obj_node, NULL);
 	zend_stack_push(&CG(short_circuiting_opnums), &jmp_null_opnum);
 }