[stable5] Added missing var that messes up IE8

This is an IE8 issue only.

Steps for the issue:
1. In the public page, select all files
2. Click "Download" in the header

Before the fix: JS error and redirect to main page
After the fix: zip download is initiated

Please review/test in IE8: @felixboehm @icewind1991 @MorrisJobke