Fix CVE-2018-6942:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-6942
https://people.canonical.com/~ubuntu-security/cve/2018/CVE-2018-6942.html

Copied from upstream (ChangeLog section removed):
https://git.savannah.gnu.org/cgit/freetype/freetype2.git/commit/?id=29c759284e305ec428703c9a5831d0b1fc3497ef

diff --git a/src/truetype/ttinterp.c b/src/truetype/ttinterp.c
index d855aaa..551f14a 100644
--- a/src/truetype/ttinterp.c
+++ b/src/truetype/ttinterp.c
@@ -7532,8 +7532,16 @@
       return;
     }
 
-    for ( i = 0; i < num_axes; i++ )
-      args[i] = coords[i] >> 2; /* convert 16.16 to 2.14 format */
+    if ( coords )
+    {
+      for ( i = 0; i < num_axes; i++ )
+        args[i] = coords[i] >> 2; /* convert 16.16 to 2.14 format */
+    }
+    else
+    {
+      for ( i = 0; i < num_axes; i++ )
+        args[i] = 0;
+    }
   }
 
 
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<div class='path'>path: <a href='/guix/log/?id=688d99282b2e4db61de6a58df903378a6b34810f'>root</a>/<a href='/guix/log/tests?id=688d99282b2e4db61de6a58df903378a6b34810f'>tests</a>/<a href='/guix/log/tests/combinators.scm?id=688d99282b2e4db61de6a58df903378a6b34810f'>combinators.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/combinators.scm?id=688d99282b2e4db61de6a58df903378a6b34810f&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2016-05-04 23:35:55 +0200'>2016-05-04</span></td><td><a href='/guix/commit/tests/combinators.scm?id=958dd3ce68733bcd5c1231424c7e4ad39e67594a'>utils: Move combinators to (guix combinators).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/utils.scm (compile-time-value, memoize, fold2)
(fold-tree, fold-tree-leaves): Move to...
* guix/combinators: ... here.  New file.
* tests/utils.scm ("fold2, 1 list", "fold2, 2 lists")
(fold-tree tests): Move to...
* tests/combinators.scm: ... here.  New file.
* Makefile.am (MODULES, SCM_TESTS): Add them.
* gnu/packages.scm, gnu/packages/bootstrap.scm,
gnu/services/herd.scm, guix/build-system/gnu.scm,
guix/build-system/python.scm, guix/derivations.scm,
guix/gnu-maintenance.scm, guix/import/elpa.scm,
guix/scripts/archive.scm, guix/scripts/build.scm,
guix/scripts/graph.scm, guix/scripts/lint.scm,
guix/scripts/size.scm, guix/scripts/substitute.scm,
guix/serialization.scm, guix/store.scm, guix/ui.scm: Adjust imports
accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>