Check that 'lossy' key is set before attempting to get its value

Fixes:

	Notice: Undefined index: lossy in
	/srv/mediawiki/php-1.25wmf6/extensions/PagedTiffHandler/PagedTiffHandler_body.php
	on line 419

Bug: 73108

Change-Id: I31a939f61ea0ae55ce32f0d9e3b5b41f4f049f2e