Fix CVE-2016-3990 (write buffer overflow in PixarLogEncode if more input
samples are provided than expected by PixarLogSetupEncode).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-3990
http://bugzilla.maptools.org/show_bug.cgi?id=2544

Patch extracted from upstream CVS repo with:
$ cvs diff -u -r1.45 -r1.46 libtiff/tif_pixarlog.c

Index: libtiff/tif_pixarlog.c
===================================================================
RCS file: /cvs/maptools/cvsroot/libtiff/libtiff/tif_pixarlog.c,v
retrieving revision 1.45
retrieving revision 1.46
diff -u -r1.45 -r1.46
--- libtiff/libtiff/tif_pixarlog.c	28 Jun 2016 15:37:33 -0000	1.45
+++ libtiff/libtiff/tif_pixarlog.c	15 Aug 2016 20:49:48 -0000	1.46
@@ -1141,6 +1141,13 @@
 	}
 
 	llen = sp->stride * td->td_imagewidth;
+    /* Check against the number of elements (of size uint16) of sp->tbuf */
+    if( n > td->td_rowsperstrip * llen )
+    {
+        TIFFErrorExt(tif->tif_clientdata, module,
+                     "Too many input bytes provided");
+        return 0;
+    }
 
 	for (i = 0, up = sp->tbuf; i < n; i += llen, up += llen) {
 		switch (sp->user_datafmt)  {