Fix CVE-2016-3623.

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-3623
http://bugzilla.maptools.org/show_bug.cgi?id=2569

Patch extracted from upstream CVS repo with:
$ cvs diff -u -r1.16 -r1.17 tools/rgb2ycbcr.c

Index: tools/rgb2ycbcr.c
===================================================================
RCS file: /cvs/maptools/cvsroot/libtiff/tools/rgb2ycbcr.c,v
retrieving revision 1.16
retrieving revision 1.17
diff -u -r1.16 -r1.17
--- libtiff/tools/rgb2ycbcr.c	21 Jun 2015 01:09:10 -0000	1.16
+++ libtiff/tools/rgb2ycbcr.c	15 Aug 2016 21:26:56 -0000	1.17
@@ -95,9 +95,13 @@
 			break;
 		case 'h':
 			horizSubSampling = atoi(optarg);
+            if( horizSubSampling != 1 && horizSubSampling != 2 && horizSubSampling != 4 )
+                usage(-1);
 			break;
 		case 'v':
 			vertSubSampling = atoi(optarg);
+            if( vertSubSampling != 1 && vertSubSampling != 2 && vertSubSampling != 4 )
+                usage(-1);
 			break;
 		case 'r':
 			rowsperstrip = atoi(optarg);
