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_cli.c b/library/ssl_cli.c
index 4fe767b..b6fe21f 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -2173,7 +2173,7 @@
              * };
              */
             if( ( ret = md_init_ctx( &ctx,
-                                     md_info_from_type( md_alg ) ) ) != 0 )
+                                     md_info_from_type( md_alg ), 0 ) ) != 0 )
             {
                 SSL_DEBUG_RET( 1, "md_init_ctx", ret );
                 return( ret );