Module API: Fail ineffective auth calls.

The client pointed to by the module context may in some cases be a fake
client. RM_Authenticate*() calls in this case would be ineffective but
appear to succeed, and this change fails them to make it easier to catch
such cases.

(cherry picked from commit 82866776d0c26f17043f9c1b0f0f5f48660e6848)

src/module.c

@@ -5638,6 +5638,11 @@ static int authenticateClientWithUser(RedisModuleCtx *ctx, user *user, RedisModu
         return REDISMODULE_ERR;
     }
 
+    /* Avoid settings which are meaningless and will be lost */
+    if (!ctx->client || (ctx->client->flags & CLIENT_MODULE)) {
+        return REDISMODULE_ERR;
+    }
+
     moduleNotifyUserChanged(ctx->client);
 
     ctx->client->user = user;