Fix CVE-2016-5321. 

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-5321
http://bugzilla.maptools.org/show_bug.cgi?id=2558

Patch extracted from upstream CVS repo with:
$ cvs diff -u -r1.35 -r1.36 tools/tiffcrop.c

Index: tools/tiffcrop.c
===================================================================
RCS file: /cvs/maptools/cvsroot/libtiff/tools/tiffcrop.c,v
retrieving revision 1.35
retrieving revision 1.36
diff -u -r1.35 -r1.36
--- libtiff/tools/tiffcrop.c	19 Aug 2015 02:31:04 -0000	1.35
+++ libtiff/tools/tiffcrop.c	11 Jul 2016 21:26:03 -0000	1.36
@@ -989,7 +989,7 @@
     nrow = (row + tl > imagelength) ? imagelength - row : tl;
     for (col = 0; col < imagewidth; col += tw)
       {
-      for (s = 0; s < spp; s++)
+      for (s = 0; s < spp && s < MAX_SAMPLES; s++)
         {  /* Read each plane of a tile set into srcbuffs[s] */
 	tbytes = TIFFReadTile(in, srcbuffs[s], col, row, 0, s);
         if (tbytes < 0  && !ignore)