Fixing minor bug with getting curve key from socket options

There was a minor bug with incorrect curve key length provided when getting it from socket config. 

In libzmq there is a check, wheter key length is equal to 32 (if yes, key in binary form is returned), or equal to to 41 (which is a length of curve key + closing 0 char, therfore Z85 encoded string is returned). 

If none of this occurs, empty pointer is being returned.