[__package_update_index] Fix maxage false-positives on brand-new installations
This merge request fixes the bug #803 (closed) by using
-1 as value if no age of the latest index update could be found. This value will be checked separately when the max age is checked.
This can be tested by removing the file which will be used to detect the age of the latest index update. On apt systems, this is done as follow:
Before the fix, it won't update the package index after the file was removed, even if
--maxage 1 is given. With the fix, the package update will be done immediately.