Description:
------------
When ImageTrueColorToPalette is called with ncolors = to zero (or an uninitialized variable), it segfaults.

Note: if $img is not loaded with a valid jpg, it will not crash, simply error on bad image resource.  So make sure 'test.jpg' points to a valid jpeg.

Reproduce code:
---------------
<?php
  $img = imagecreatefromjpeg('test.jpg');
  ImageTrueColorToPalette($img, false, 0);
?>

Expected result:
----------------
No expected output, simply $img should be an indexed image.

Actual result:
--------------
Segmentation fault