Fix buffer size parameter for exported keying material.

Fix buffer size parameter for exported keying material.

Commit 41e4b67a229e774ebc57a882c386e10d80e10e7e broke the exported
keying material functionality while addressing lack of variable-length
arrays in MSVC compilers - turning an array into a gc_malloc()'ed
pointer, but still using "sizeof(ekm)" for buffer size - which is
now "4" (unsigned char *), not the actual buffer length...

Fixed!

Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <49496.109.81.184.65.1461736834.squirrel@mail.actumg2.cz>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11509

Signed-off-by: Gert Doering <gert@greenie.muc.de>