Fix keymgr bug wrt setting the next time

Only set the next time the keymgr should run if the value is non zero.
Otherwise we default back to one hour. This may happen if there is one
or more key with an unlimited lifetime.

lib/dns/keymgr.c

@@ -1777,7 +1777,9 @@ keymgr_key_rollover(dns_kasp_key_t *kaspkey, dns_dnsseckey_t *active_key,
 		if (prepub == 0 || prepub > now) {
 			/* No need to start rollover now. */
 			if (*nexttime == 0 || prepub < *nexttime) {
-				*nexttime = prepub;
+				if (prepub > 0) {
+					*nexttime = prepub;
+				}
 			}
 			return ISC_R_SUCCESS;
 		}