* base64.c: Add new members READER and WRITER in union U2.
(gpgsm_create_reader): Initialise CTX->u2.reader.
(gpgsm_destroy_reader): Invoke ksba_reader_release. Return early
if CTX is NULL.
(gpgsm_create_writer): Initialise CTX->u2.writer.
(gpgsm_destroy_writer): Invoke ksba_writer_release. Return early
if CTX is NULL.