Possible ob_get_clean() failure

Under certain conditions, minifyhtml fails to get and drop the output buffer and may output garbage. Error is logged:

Notice : ob_get_clean(): failed to delete buffer of zlib output compression (1) dans minifyhtml_exit() (ligne 38 dans .../minifyhtml/minifyhtml.module).

To reproduce, set zlib.output_compression = On in php.ini, enable html minification, install and configure module xmlsitemap and browse to the sitemap URL. The result is the log above and a corrupt XML output.

The attached patch fixes it by giving up with minification if some buffer error occurs.