knownhosts: Make sure we have both knownhosts files ready
If either one is missing at this point, fill it with default vaules in
Previously, when setting up only knownhosts, global_knownhosts file
was left pointing to NULL and the ssh_known_hosts_read_entries()
was trying to open NULL file which is invalid.
Signed-off-by: Jakub Jelen <email@example.com>
Reviewed-by: Andreas Schneider <firstname.lastname@example.org>