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_tls.c b/library/ssl_tls.c
index 2a61a51..ded226e 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -658,8 +658,8 @@
         int ret;
 
         /* Initialize HMAC contexts */
-        if( ( ret = md_init_ctx( &transform->md_ctx_enc, md_info ) ) != 0 ||
-            ( ret = md_init_ctx( &transform->md_ctx_dec, md_info ) ) != 0 )
+        if( ( ret = md_init_ctx( &transform->md_ctx_enc, md_info, 1 ) ) != 0 ||
+            ( ret = md_init_ctx( &transform->md_ctx_dec, md_info, 1 ) ) != 0 )
         {
             SSL_DEBUG_RET( 1, "md_init_ctx", ret );
             return( ret );