Description:
------------
If very large images (i.e. >= 256 megapixels) are passed to
imagewbmp(), the function causes a segfault since createwbmp()[1]
fails, but gdImageWBMPCtx() continues after raising an error[2].
This issue does not affect upstream libgd, where this issue has
been fixed long ago with commit 46fd625[3].

It seems to me that this is a low security issue (maybe even no
security issue at all), since it makes no sense to have such large
WBMP images at all, and usually PHP's memory_limit is set low
enough to prevent to allocate such large images at all.

[1] <https://github.com/php/php-src/blob/php-7.1.26/ext/gd/libgd/wbmp.c#L110-L140>
[2] <https://github.com/php/php-src/blob/php-7.1.26/ext/gd/libgd/gd_wbmp.c#L100-L102>
[3] <https://github.com/libgd/libgd/commit/46fd62585ecbced255193cd5fe964bc44eb137de>

Test script:
---------------
<?php
$im = imagecreate(40000, 20000);
imagecolorallocate($im, 0, 0, 0);
imagewbmp($im, __DIR__ . '/wbmp.wbmp');
echo "DONE\n";

Expected result:
----------------
Warning: imagewbmp(): gd warning: product of memory allocation multiplication would exceed INT_MAX, failing operation gracefully
 in %s on line %d

Warning: imagewbmp(): Could not create WBMP in %s on line %d
DONE

Actual result:
--------------
Warning: imagewbmp(): gd warning: product of memory allocation multiplication would exceed INT_MAX, failing operation gracefully
 in %s on line %d

Warning: imagewbmp(): Could not create WBMP in %s on line %d
Segmentation fault (core dumped)