Fix #67325: imagetruecolortopalette: white is duplicated in palette

gdImageTrueColorToPalette() is sometimes wasteful by putting multiple white
color entries into the palette. This is caused by an obvious typo, where
to avoid a division by zero when `total` is zero, `count` is checked instead
of `total`.

We fix this issue to improve the quality of the color quantization.

Cf. <libgd/libgd@24b4550f>

Author: cmb69

NEWS

@@ -2,6 +2,10 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? 2016, PHP 5.6.27
 
+- GD:
+  . Fixed bug #67325 (imagetruecolortopalette: white is duplicated in palette).
+    (cmb)
+
 15 Sep 2016, PHP 5.6.26
 
 - Core:

ext/gd/libgd/gd_topal.c

@@ -760,7 +760,7 @@ LOCAL (void)
   cinfo->colormap[2][icolor] = (JSAMPLE) ((c2total + (total >> 1)) / total);
 #else
   /* 2.0.16: Paul den Dulk found an occasion where total can be 0 */
-  if (count)
+  if (total)
     {
       nim->red[icolor] = (int) ((c0total + (total >> 1)) / total);
       nim->green[icolor] = (int) ((c1total + (total >> 1)) / total);

ext/gd/tests/bug67325.jpeg

@@ -0,0 +1,29 @@
+--TEST--
+Bug #67325 (imagetruecolortopalette: white is duplicated in palette)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+if (!(imagetypes() & IMG_JPG)) die('skip JPEG support not available');
+?>
+--FILE--
+<?php
+$filename = __DIR__ . DIRECTORY_SEPARATOR . 'bug67325.jpeg';
+
+$im = imagecreatefromjpeg($filename);
+imagetruecolortopalette($im, 0, 256);
+
+$white = 0;
+for ($i = 0; $i < 256; $i++) {
+    $components = imagecolorsforindex($im, $i);
+    if ($components['red'] === 255 && $components['green'] === 255 && $components['blue'] === 255) {
+        $white++;
+    }
+}
+var_dump($white);
+
+imagedestroy($im);
+?>
+===DONE===
+--EXPECT--
+int(0)
+===DONE===