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;
+    }
   }