Fix CVE-2017-9936:

http://bugzilla.maptools.org/show_bug.cgi?id=2706
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-9936
https://security-tracker.debian.org/tracker/CVE-2017-9936

Patch lifted from upstream source repository (the changes to 'ChangeLog'
don't apply to the libtiff 4.0.8 release tarball).

3rd party Git reference:

https://github.com/vadz/libtiff/commit/fe8d7165956b88df4837034a9161dc5fd20cf67a

2017-06-26  Even Rouault <even.rouault at spatialys.com>

        * libtiff/tif_jbig.c: fix memory leak in error code path of
JBIGDecode()
        Fixes http://bugzilla.maptools.org/show_bug.cgi?id=2706
        Reported by team OWL337

/cvs/maptools/cvsroot/libtiff/ChangeLog,v  <--  ChangeLog
new revision: 1.1254; previous revision: 1.1253
/cvs/maptools/cvsroot/libtiff/libtiff/tif_jbig.c,v  <--  libtiff/tif_jbig.c
new revision: 1.16; previous revision: 1.15

Index: libtiff/libtiff/tif_jbig.c
===================================================================
RCS file: /cvs/maptools/cvsroot/libtiff/libtiff/tif_jbig.c,v
retrieving revision 1.15
retrieving revision 1.16
diff -u -r1.15 -r1.16
--- libtiff/libtiff/tif_jbig.c	10 Mar 2010 18:56:48 -0000	1.15
+++ libtiff/libtiff/tif_jbig.c	26 Jun 2017 15:20:00 -0000	1.16
@@ -1,4 +1,4 @@
-/* $Id: tif_jbig.c,v 1.15 2010-03-10 18:56:48 bfriesen Exp $ */
+/* $Id: tif_jbig.c,v 1.16 2017-06-26 15:20:00 erouault Exp $ */
 
 /*
  * Copyright (c) 1988-1997 Sam Leffler
@@ -94,6 +94,7 @@
 			     jbg_strerror(decodeStatus)
 #endif
 			     );
+		jbg_dec_free(&decoder);
 		return 0;
 	}