client: prohibit endpoint ids without @ character

The function mgcp_msg_gen() does only check if the user supplied
an endpoint name or not. The user may still supply an endpoint
name that does not contain the separator (@) character.

- Refuse to generate the message if the endpoint name does not
  contain any @ character.

Change-Id: I92dd1556e4a26b4bef8e1c8c57141552abf988ca
1 file changed