Fix CVE-2016-6525 (heap overflow in pdf_load_mesh_params()).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-6525
https://security-tracker.debian.org/tracker/CVE-2016-6525

Patch copied from upstream source repository:
http://git.ghostscript.com/?p=mupdf.git;h=39b0f07dd960f34e7e6bf230ffc3d87c41ef0f2e

diff --git a/source/pdf/pdf-shade.c b/source/pdf/pdf-shade.c
index 7815b3c..6e25efa 100644
--- a/source/pdf/pdf-shade.c
+++ b/source/pdf/pdf-shade.c
@@ -206,7 +206,7 @@ pdf_load_mesh_params(fz_context *ctx, pdf_document *doc, fz_shade *shade, pdf_ob
 	obj = pdf_dict_get(ctx, dict, PDF_NAME_Decode);
 	if (pdf_array_len(ctx, obj) >= 6)
 	{
-		n = (pdf_array_len(ctx, obj) - 4) / 2;
+		n = fz_mini(FZ_MAX_COLORS, (pdf_array_len(ctx, obj) - 4) / 2);
 		shade->u.m.x0 = pdf_to_real(ctx, pdf_array_get(ctx, obj, 0));
 		shade->u.m.x1 = pdf_to_real(ctx, pdf_array_get(ctx, obj, 1));
 		shade->u.m.y0 = pdf_to_real(ctx, pdf_array_get(ctx, obj, 2));
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<tr><td><span title='2021-05-31 10:45:53 +0200'>2021-05-31</span></td><td><a href='/guix/commit/gnu/packages/sequoia.scm?id=85d80283b6d14b19dacdd687590eba494da4864f'>gnu: pep-engine: Update to 2.1.34.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pep.scm (fdik-libetpan): Update to current development
  version. [source]: Switch to pep.foundation.
  (pep-engine): Update to 2.1.34. [source]: Switch to new git repo.
  [arguments]&lt;make-flags&gt;: Enable release-build.
  [inputs]: Use normal sequoia.
* gnu/packages/sequoia.scm (sequoia4pEp): Remove variable.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2021-05-23 17:49:56 +0200'>2021-05-23</span></td><td><a href='/guix/commit/gnu/packages/sequoia.scm?id=dbcf2b61b1f48cbe84d51873f4e4de60897b914c'>gnu: Fix erroneous uses of 'package/inherit'.</a><span class='msg-avail'>...<span class='msg-tooltip'>These packages use a different source/version from the inherited package, and
thus should not inherit replacements.

* gnu/packages/check.scm (googletest-1.8, python-pytest-runner-2): Use record
inheritance instead of PACKAGE/INHERIT.
* gnu/packages/crates-io.scm (rust-bindgen-0.49, rust-bytes-0.4,
rust-colored-1.9.1, rust-difference-1, rust-docopt-0.8, rust-html5ever-0.23,
rust-http-0.1, rust-http-body-0.1, rust-loom-0.2, rust-loom-0.1,
rust-pulldown-cmark-0.0.8, rust-ring-0.13, rust-rustls-0.12, rust-sct-0.3,
rust-term-0.2, rust-untrusted-0.6, rust-webpki-0.18, rust-webpki-roots-0.17,
rust-webpki-roots-0.14): Likewise.
* gnu/packages/emacs.scm (emacs-next, emacs-next-pgtk, guile-emacs): Likewise.
* gnu/packages/guile.scm (guile-2.2.4): Likewise.
* gnu/packages/maths.scm (hdf5-1.10, hdf5-1.12): Likewise.
* gnu/packages/protobuf.scm (protobuf-3.6, protobuf-3.5, python-protobuf-3.6):
Likewise.
* gnu/packages/python-web.scm (python2-html2text): Likewise.
* gnu/packages/python-xyz.scm (python-pygments/fixed, python2-numpy,
python2-urwid, python2-markdown, python2-fonttools): Likewise.
* gnu/packages/racket.scm (racket): Likewise.
* gnu/packages/sequoia.scm (sequoia4pEp): Likewise.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2021-05-04 09:44:14 +0200'>2021-05-04</span></td><td><a href='/guix/commit/gnu/packages/sequoia.scm?id=ec4012c5820fb139d07dee492f8b0d492ec1c042'>gnu: sequoia: Update to 1.1.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sequoia.scm (sequoia): Update to 1.1.0. [arguments]: Remove
  phase "package", add phases "fix-rand-dependency" and "fix-permissions".
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2021-03-22 11:20:17 +0200'>2021-03-22</span></td><td><a href='/guix/commit/gnu/packages/sequoia.scm?id=ff1c7e40252337cb2c992c2d41e0ed4cefe56df0'>gnu: sequoia4pEp: Adjust inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sequoia.scm (sequoia4pEp)[arguments]: Override inherited
cargo-inputs, cargo-development-inputs. Replace inherited 'unpin-deps
phase.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2021-03-22 09:54:19 +0200'>2021-03-22</span></td><td><a href='/guix/commit/gnu/packages/sequoia.scm?id=4d0fe40f65e096a4d07f4cc8c95139a4dda96c53'>gnu: sequoia4pEp: Move to sequoia.scm.</a><span class='msg-avail'>...<span class='msg-tooltip'>Don't have cross module package inheritance.

* gnu/packages/pep.scm (sequoia4pEp): Move to ...
* gnu/packages/sequoia.scm (sequoia4pEp): ... here. Hide package.
</span></span></td><td>Efraim Flashner</td></tr>