m3ua: m3ua_gen_error_msg(): include Routing Context IE in cause Invalid Routing Context
As per RFC4666 3.8.1: """ The "Invalid Routing Context" error is sent if a message is received from a peer with an invalid (unconfigured) Routing Context value. For this error, the invalid Routing Context(s) MUST be included in the Error message." """
Hence add the originating Routing Context IE so the error message can be further originated. For the same reason, also add the Diagnostic Information for the specific cases in the switch statement, and make them more robust about possible null pointer dereference.
