g10: Minor clean up for export.c.

* g10/export.c (export_ssh_key): Check IDENTIFIER for error.
Release base64 thing on error of get_membuf.

Compiler (older) may misunderstand the variable IDENTIFIER is not
initialized, while good one can do better analysys on the value for
ERR (and thus, IDENTIFIER).

On the error of get_membuf, still, b64enc_finish should be called,
even if it lost the ERR value.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>