[CVE-2020-7595] Fix infinite loop in xmlStringLenDecodeEntities

[CVE-2020-7595] Fix infinite loop in xmlStringLenDecodeEntities

When ctxt->instate == XML_PARSER_EOF,xmlParseStringEntityRef
return NULL which cause a infinite loop in xmlStringLenDecodeEntities

Found with libFuzzer.

From 0e1a49c8907645d2e155f0d89d4d9895ac5112b5 Mon Sep 17 00:00:00 2001
From: Zhipeng Xie <xiezhipeng1@huawei.com>
Date: Thu, 12 Dec 2019 17:30:55 +0800

Change-Id: Icf48083013a7e949a591eac94dc6def067a49459
Signed-off-by: Zhipeng Xie <xiezhipeng1@huawei.com>
Signed-off-by: JinWang An <jinwang.an@samsung.com>