don't delete floodfill if number of remaining floodfills is less than minimal

(cherry picked from commit 541464b705)

libi2pd/NetDb.hpp

@@ -36,6 +36,7 @@ namespace i2p
 namespace data
 {
 	const int NETDB_MIN_ROUTERS = 90;
+	const int NETDB_MIN_FLOODFILLS = 5;
 	const int NETDB_FLOODFILL_EXPIRATION_TIMEOUT = 60 * 60; // 1 hour, in seconds
 	const int NETDB_INTRODUCEE_EXPIRATION_TIMEOUT = 65 * 60;
 	const int NETDB_MIN_EXPIRATION_TIMEOUT = 90 * 60; // 1.5 hours