Don't try to deref a pointer with cffi when doing a null check

This causes issues with pointers to opaque structs

Author: wbond

oscrypto/_ffi.py

@@ -111,8 +111,6 @@ def is_null(point):
             return True
         if ffi.getctype(ffi.typeof(point)) == 'void *':
             return False
-        if point[0] == ffi.NULL:
-            return True
         return False
 
     def errno():