location-importer: Only delete override data if we are sure to have a valid replacement

location-importer: Only delete override data if we are sure to have a valid replacement

The current way of truncating all override data straight away leaves us
with no data at all, should a source turn out to be unreachable or
returning bogus files (yes, Cloudflare, I _am_ looking at you).

It is therefore better to only delete data we know to have a valid
replacement for, rather than just dropping the source altogether.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>