Render 404 instead of 500 if trying to see snippets of unexisting user

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/snippets_controller.rb

@@ -19,6 +19,9 @@ class SnippetsController < ApplicationController
 
   def user_index
     @user = User.find_by(username: params[:username])
+
+    render_404 and return unless @user
+
     @snippets = @user.snippets.fresh.non_expired
 
     if @user == current_user