use (cffi:null-pointer) instead of 0 in call to ssl-ctx-ctrl

 * the change from :long to :pointer requires that we use a proper null pointer instead of 0.
Merge pull request #4 from slyrus/ssl-ctx-ctrl-pointer-fix

use (cffi:null-pointer) instead of 0 in call to ssl-ctx-ctrl
re #4 - use (cffi:null-pointer) instead of 0 in call to ssl-ctx-ctrl