Fix potential NULL pointer dereference in gdImageClone()

[fcabralpacheco]

This PR fix a bug in the gdImageClone(), which can be used to generate a DoS condition on servers running PHP linked with the affected versions.

This patch has been improved (with the addition of tests) by @cmb69 .

CVE assigned: CVE-2018-14553

Original patch description:

`From 1e9d4d8a14619b7721cf1fc48a2260c24457418a Mon Sep 17 00:00:00 2001
From: "Christoph M. Becker" cmbecker69@gmx.de
Date: Fri, 20 Jul 2018 10:59:01 +0200
Subject: [PATCH] Fix potential NULL pointer dereference in gdImageClone()

If the image which should be cloned has set any style, the current code
which copies the style manually would dereference a NULL pointer. We
can simply remove this manual copying, since a few lines later in the
function we're already calling gdImageSetStyle() to copy the style
data. We have to make sure, though, to appropriately set the stylePos.

This security bug has been reported by Fabio Cabral Pacheco who also
provided the basic patch.`

[willson-chen]

@cmb69 hi, could you please review this PR?

@fcabralpacheco Are you sure that the CVE ID(CVE-2018-14553) is assigned to libgd? I find it reserved in CVE Community. Could you please drop me a link?

[fcabralpacheco]

@willson-chen yes, I requested the CVE, I will ask to update and make it public.

Thanks.

[fcabralpacheco]

@willson-chen CVE update requested.

[cmb69]

Oops, I totally forgot about this one. Thanks for the reminder! Patch is fine; applied as a93eac0.