journalctl: fix --update-catalog with not --root arg

grawity> `journalctl --update-catalog` from latest git prints:
         "Recursive mkdir .: Invalid argument" and
         "Failed to write : Invalid argument"
Always use errno > 0 to help gcc

gcc thinks that errno might be negative, and functions could return
something positive on error (-errno). Should not matter in practice,
but makes an -O4 build much quieter.