update: do not crash on screenshots without texts/graphics

`fdroid update` crashed for apps that only had screenshots but no graphics
or localized texts because destdir was not being set in that case.  This
fixes that and adds a test case.

closes #320
!286

fdroidserver/update.py

@@ -716,6 +716,7 @@ def insert_localized_app_metadata(apps):
                 logging.debug(packageName + ' does not have app metadata, skipping l18n scan.')
                 continue
             locale = segments[-1]
+            destdir = os.path.join('repo', packageName, locale)
             for f in files:
                 if f in ('description.txt', 'full_description.txt'):
                     _set_localized_text_entry(apps[packageName], locale, 'description',