znc: denial of service

From the Red Hat bugzilla entry:

An out-of-range flaw was found in znc where if it received a "PING" from a client without an argument, std::string would throw a std::out_of_range exception which killed znc.

Some unsafe substr() calls were fixed as well. These are of lesser impact because a valid login is required in order to cause a std::out_of_range exception.