Fix CVE-2016-3991 (out-of-bounds write in loadImage()).

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-3991
http://bugzilla.maptools.org/show_bug.cgi?id=2543

Patch extracted from upstream CVS repo with:
$ cvs diff -u -r1.37 -r1.38 tools/tiffcrop.c

Index: tools/tiffcrop.c
===================================================================
RCS file: /cvs/maptools/cvsroot/libtiff/tools/tiffcrop.c,v
retrieving revision 1.37
retrieving revision 1.38
diff -u -r1.37 -r1.38
--- libtiff/tools/tiffcrop.c	11 Jul 2016 21:38:31 -0000	1.37
+++ libtiff/tools/tiffcrop.c	15 Aug 2016 21:05:40 -0000	1.38
@@ -798,6 +798,11 @@
     }
 
   tile_buffsize = tilesize;
+  if (tilesize == 0 || tile_rowsize == 0)
+  {
+     TIFFError("readContigTilesIntoBuffer", "Tile size or tile rowsize is zero");
+     exit(-1);
+  }
 
   if (tilesize < (tsize_t)(tl * tile_rowsize))
     {
@@ -807,7 +812,12 @@
               tilesize, tl * tile_rowsize);
 #endif
     tile_buffsize = tl * tile_rowsize;
-    } 
+    if (tl != (tile_buffsize / tile_rowsize))
+    {
+    	TIFFError("readContigTilesIntoBuffer", "Integer overflow when calculating buffer size.");
+        exit(-1);
+    }
+    }
 
   tilebuf = _TIFFmalloc(tile_buffsize);
   if (tilebuf == 0)
@@ -1210,6 +1220,12 @@
       !TIFFGetField(out, TIFFTAG_BITSPERSAMPLE, &bps) )
       return 1;
 
+  if (tilesize == 0 || tile_rowsize == 0 || tl == 0 || tw == 0)
+  {
+    TIFFError("writeBufferToContigTiles", "Tile size, tile row size, tile width, or tile length is zero");
+    exit(-1);
+  }
+  
   tile_buffsize = tilesize;
   if (tilesize < (tsize_t)(tl * tile_rowsize))
     {
@@ -1219,6 +1235,11 @@
               tilesize, tl * tile_rowsize);
 #endif
     tile_buffsize = tl * tile_rowsize;
+    if (tl != tile_buffsize / tile_rowsize)
+    {
+	TIFFError("writeBufferToContigTiles", "Integer overflow when calculating buffer size");
+	exit(-1);
+    }
     }
 
   tilebuf = _TIFFmalloc(tile_buffsize);
@@ -5945,12 +5966,27 @@
     TIFFGetField(in, TIFFTAG_TILELENGTH, &tl);
 
     tile_rowsize  = TIFFTileRowSize(in);      
+    if (ntiles == 0 || tlsize == 0 || tile_rowsize == 0)
+    {
+	TIFFError("loadImage", "File appears to be tiled, but the number of tiles, tile size, or tile rowsize is zero.");
+	exit(-1);
+    }
     buffsize = tlsize * ntiles;
+    if (tlsize != (buffsize / ntiles))
+    {
+	TIFFError("loadImage", "Integer overflow when calculating buffer size");
+	exit(-1);
+    }
 
-        
     if (buffsize < (uint32)(ntiles * tl * tile_rowsize))
       {
       buffsize = ntiles * tl * tile_rowsize;
+      if (ntiles != (buffsize / tl / tile_rowsize))
+      {
+	TIFFError("loadImage", "Integer overflow when calculating buffer size");
+	exit(-1);
+      }
+      
 #ifdef DEBUG2
       TIFFError("loadImage",
 	        "Tilesize %u is too small, using ntiles * tilelength * tilerowsize %lu",
@@ -5969,8 +6005,25 @@
     TIFFGetFieldDefaulted(in, TIFFTAG_ROWSPERSTRIP, &rowsperstrip);
     stsize = TIFFStripSize(in);
     nstrips = TIFFNumberOfStrips(in);
+    if (nstrips == 0 || stsize == 0)
+    {
+	TIFFError("loadImage", "File appears to be striped, but the number of stipes or stripe size is zero.");
+	exit(-1);
+    }
+
     buffsize = stsize * nstrips;
-    
+    if (stsize != (buffsize / nstrips))
+    {
+	TIFFError("loadImage", "Integer overflow when calculating buffer size");
+	exit(-1);
+    }
+    uint32 buffsize_check;
+    buffsize_check = ((length * width * spp * bps) + 7);
+    if (length != ((buffsize_check - 7) / width / spp / bps))
+    {
+	TIFFError("loadImage", "Integer overflow detected.");
+	exit(-1);
+    }
     if (buffsize < (uint32) (((length * width * spp * bps) + 7) / 8))
       {
       buffsize =  ((length * width * spp * bps) + 7) / 8;