posts: fix exception when viewing invalid .zip file.

Fix exception when viewing post #23077 (a .zip that is not a ugoira).
Caused by including large_image_width in the image's data-* attributes.
The image_width was nil, which caused a comparison to fail.

app/models/post.rb

@@ -239,9 +239,9 @@ class Post < ApplicationRecord
 
     def large_image_width
       if has_large?
-        [Danbooru.config.large_image_width, image_width].min
+        [Danbooru.config.large_image_width, image_width.to_i].min
       else
-        image_width
+        image_width.to_i
       end
     end