knownhosts: Make sure we have both knownhosts files ready

If either one is missing at this point, fill it with default vaules in
ssh_options_apply().

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 <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 5159cd96e8)

src/knownhosts.c

@@ -306,7 +306,8 @@ struct ssh_list *ssh_known_hosts_get_algorithms(ssh_session session)
     int list_error = 0;
     int rc;
 
-    if (session->opts.knownhosts == NULL) {
+    if (session->opts.knownhosts == NULL ||
+        session->opts.global_knownhosts == NULL) {
         if (ssh_options_apply(session) < 0) {
             ssh_set_error(session,
                           SSH_REQUEST_DENIED,