posts: fix exception when viewing post with source Blog..

Fix a PublicSuffix::DomainNotAllowed exception raised with viewing or editing a post
with a source like `Blog.`.

This happened when parsing the post's source. `Danbooru::URL.parse("Blog.")` would
heuristically parse the source into `http://blog`. Calling any methods related to the
URL's hostname or domain would lead to calling `PublicSuffix.parse("blog")`, which
would fail with PublicSuffix::DomainNotAllowed.

app/logical/danbooru/url.rb

@@ -45,6 +45,7 @@ module Danbooru
       @url.path = nil if @url.path == "/"
 
       raise Error, "#{original_url} is not an http:// URL" if !@url.normalized_scheme.in?(["http", "https"])
+      raise Error, "#{original_url} is not a valid hostname" if parsed_domain.nil?
     rescue Addressable::URI::InvalidURIError => e
       raise Error, e
     end
@@ -113,6 +114,8 @@ module Danbooru
     # @return [PublicSuffix::Domain]
     def parsed_domain
       @parsed_domain ||= PublicSuffix.parse(host)
+    rescue PublicSuffix::DomainNotAllowed
+      nil
     end
   end
 end