Fix a heap buffer overflow:

http://bugzilla.maptools.org/show_bug.cgi?id=2587

Patch extracted from upstream CVS repo:

2016-11-10 Even Rouault <even.rouault at spatialys.com>

revision 1.37
date: 2016-11-09 18:00:49 -0500;  author: erouault;  state: Exp;  lines: +10 -1;  commitid: pzKipPxDJO2dxvtz;
* libtiff/tif_strip.c: make TIFFNumberOfStrips() return the td->td_nstrips
value when it is non-zero, instead of recomputing it. This is needed in
TIFF_STRIPCHOP mode where td_nstrips is modified. Fixes a read outsize of
array in tiffsplit (or other utilities using TIFFNumberOfStrips()).
Fixes http://bugzilla.maptools.org/show_bug.cgi?id=2587

Index: libtiff/tif_strip.c
===================================================================
RCS file: /cvs/maptools/cvsroot/libtiff/libtiff/tif_strip.c,v
retrieving revision 1.36
retrieving revision 1.37
diff -u -r1.36 -r1.37
--- a/libtiff/tif_strip.c	7 Jun 2015 22:35:40 -0000	1.36
+++ b/libtiff/tif_strip.c	9 Nov 2016 23:00:49 -0000	1.37
@@ -63,6 +63,15 @@
 	TIFFDirectory *td = &tif->tif_dir;
 	uint32 nstrips;
 
+    /* If the value was already computed and store in td_nstrips, then return it,
+       since ChopUpSingleUncompressedStrip might have altered and resized the
+       since the td_stripbytecount and td_stripoffset arrays to the new value
+       after the initial affectation of td_nstrips = TIFFNumberOfStrips() in
+       tif_dirread.c ~line 3612.
+       See http://bugzilla.maptools.org/show_bug.cgi?id=2587 */
+    if( td->td_nstrips )
+        return td->td_nstrips;
+
 	nstrips = (td->td_rowsperstrip == (uint32) -1 ? 1 :
 	     TIFFhowmany_32(td->td_imagelength, td->td_rowsperstrip));
 	if (td->td_planarconfig == PLANARCONFIG_SEPARATE)