Checkmk Werk 15478: Fix possible error in availability

ID: 15478 Title: Fix possible error in availability Component: Multisite Level: 1 Class: Bug fix Version: 2.1.0p27 Since 2.1.0p25 the availabilty could show an error like "Internal error: local variable 'this_until_time' referenced before assignment" if you used a time range other than "Last month".