Fixed callproc return value refcount

Temporary anyway: I want to go back returning a list (or dict).

psycopg/cursor_type.c

@@ -1140,6 +1140,7 @@ psyco_curs_callproc(cursorObject *self, PyObject *args)
             self, operation, pvals, self->conn->async, 0)) {
 
         /* return None from this until it's DBAPI compliant... */
+        Py_INCREF(Py_None);
         res = Py_None;
     }