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_srv.c b/library/ssl_srv.c
index 250f95f..49e9b5c 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -3073,7 +3073,7 @@
              *     ServerDHParams params;
              * };
              */
-            if( ( ret = md_init_ctx( &ctx, md_info ) ) != 0 )
+            if( ( ret = md_init_ctx( &ctx, md_info, 0 ) ) != 0 )
             {
                 SSL_DEBUG_RET( 1, "md_init_ctx", ret );
                 return( ret );