polarssl: remove code duplication in key_state_write_plaintext{, _const}()

polarssl: remove code duplication in key_state_write_plaintext{, _const}()

Both functions had almost exactly the same code.  Instead of the code
duplication, have key_state_write_plaintext() call
key_state_write_plaintext_const() to do the actual work.  This is just
a bit of cleanup, it should not change any behaviour.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1430654761-26563-2-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9640
Signed-off-by: Gert Doering <gert@greenie.muc.de>