Make hmac_ctx optional

Note from future self: actually md_init_ctx will be re-introduced with the
same signature later, and a new function with the additional argument will be
added.
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index c2fde82..2b6bdc5 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -104,7 +104,7 @@
     if( ( ret = f_rng( p_rng, key, sizeof( key ) ) ) != 0 )
         return( ret );
 
-    ret = md_init_ctx( &ctx->hmac_ctx, md_info_from_type( COOKIE_MD ) );
+    ret = md_init_ctx( &ctx->hmac_ctx, md_info_from_type( COOKIE_MD ), 1 );
     if( ret != 0 )
         return( ret );