convert-snort: Fix logic for detecting enough free disk space.

convert-snort: Fix logic for detecting enough free disk space.

The subfunction only will return something if the check fails - so the logic
of the if statement was wrong set and the downloader only was called if
this check failed and to less diskspace would be available.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>