Fix CVE-2016-3945 (integer overflow in size of allocated
buffer, when -b mode is enabled, that could result in out-of-bounds
write).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-3945
http://bugzilla.maptools.org/show_bug.cgi?id=2545

Patch extracted from upstream CVS repo with:
$ cvs diff -u -r1.21 -r1.22 tools/tiff2rgba.c

Index: tools/tiff2rgba.c
===================================================================
RCS file: /cvs/maptools/cvsroot/libtiff/tools/tiff2rgba.c,v
retrieving revision 1.21
retrieving revision 1.22
diff -u -r1.21 -r1.22
--- libtiff/tools/tiff2rgba.c	21 Jun 2015 01:09:10 -0000	1.21
+++ libtiff/tools/tiff2rgba.c	15 Aug 2016 20:06:41 -0000	1.22
@@ -147,6 +147,7 @@
     uint32  row, col;
     uint32  *wrk_line;
     int	    ok = 1;
+    uint32  rastersize, wrk_linesize;
 
     TIFFGetField(in, TIFFTAG_IMAGEWIDTH, &width);
     TIFFGetField(in, TIFFTAG_IMAGELENGTH, &height);
@@ -163,7 +164,13 @@
     /*
      * Allocate tile buffer
      */
-    raster = (uint32*)_TIFFmalloc(tile_width * tile_height * sizeof (uint32));
+    rastersize = tile_width * tile_height * sizeof (uint32);
+    if (tile_width != (rastersize / tile_height) / sizeof( uint32))
+    {
+	TIFFError(TIFFFileName(in), "Integer overflow when calculating raster buffer");
+	exit(-1);
+    }
+    raster = (uint32*)_TIFFmalloc(rastersize);
     if (raster == 0) {
         TIFFError(TIFFFileName(in), "No space for raster buffer");
         return (0);
@@ -173,7 +180,13 @@
      * Allocate a scanline buffer for swapping during the vertical
      * mirroring pass.
      */
-    wrk_line = (uint32*)_TIFFmalloc(tile_width * sizeof (uint32));
+    wrk_linesize = tile_width * sizeof (uint32);
+    if (tile_width != wrk_linesize / sizeof (uint32))
+    {
+        TIFFError(TIFFFileName(in), "Integer overflow when calculating wrk_line buffer");
+	exit(-1);
+    }
+    wrk_line = (uint32*)_TIFFmalloc(wrk_linesize);
     if (!wrk_line) {
         TIFFError(TIFFFileName(in), "No space for raster scanline buffer");
         ok = 0;
@@ -249,6 +262,7 @@
     uint32  row;
     uint32  *wrk_line;
     int	    ok = 1;
+    uint32  rastersize, wrk_linesize;
 
     TIFFGetField(in, TIFFTAG_IMAGEWIDTH, &width);
     TIFFGetField(in, TIFFTAG_IMAGELENGTH, &height);
@@ -263,7 +277,13 @@
     /*
      * Allocate strip buffer
      */
-    raster = (uint32*)_TIFFmalloc(width * rowsperstrip * sizeof (uint32));
+    rastersize = width * rowsperstrip * sizeof (uint32);
+    if (width != (rastersize / rowsperstrip) / sizeof( uint32))
+    {
+	TIFFError(TIFFFileName(in), "Integer overflow when calculating raster buffer");
+	exit(-1);
+    }
+    raster = (uint32*)_TIFFmalloc(rastersize);
     if (raster == 0) {
         TIFFError(TIFFFileName(in), "No space for raster buffer");
         return (0);
@@ -273,7 +293,13 @@
      * Allocate a scanline buffer for swapping during the vertical
      * mirroring pass.
      */
-    wrk_line = (uint32*)_TIFFmalloc(width * sizeof (uint32));
+    wrk_linesize = width * sizeof (uint32);
+    if (width != wrk_linesize / sizeof (uint32))
+    {
+        TIFFError(TIFFFileName(in), "Integer overflow when calculating wrk_line buffer");
+	exit(-1);
+    }
+    wrk_line = (uint32*)_TIFFmalloc(wrk_linesize);
     if (!wrk_line) {
         TIFFError(TIFFFileName(in), "No space for raster scanline buffer");
         ok = 0;