Description:
------------
imagepng *crashes* when no color was alocated.

I see no point in rendering blank images, but this functions behavior is buggy. 

All other functions (imagejpeg, imagegif) AND imagepng on true scale images return black image. This is most likely expected result.

Instead, this function flushes some part of header and then crashes with warnings when trying to convert non existing color palette.

GD Version  bundled (2.0.34 compatible)  

Reproduce code:
---------------
<?
$img = imagecreate(500,500);
//header("Content-type: image/png"); // skip this to see the problem, else browser will ignore corrupted image.
imagepng($img);
imagedestroy($img);
?>

Expected result:
----------------
blank black image (like other image* functions) or E_WARNING without junk.

Actual result:
--------------
?PNG  IHDR
?
?
?M?
Warning: imagepng() [function.imagepng]: gd-png: fatal libpng error: Invalid number of colors in palette in C:\usr\apache\httpd\html\draw.php on line 21

Warning: imagepng() [function.imagepng]: gd-png error: setjmp returns error condition in C:\usr\apache\httpd\html\draw.php on line 21