FIX: generate_topic_thumbnails job infinitely running for corrupted images

It's possible that the original topic image is broken in some form, so
we shouldn't try and generate a topic thumbnail for it. The fix will
prevent the generate_topic_thumbnails job being enqueued every time the
topic is viewed.

app/models/topic.rb

@@ -115,6 +115,8 @@ class Topic < ActiveRecord::Base
         image_upload &&
         SiteSetting.create_thumbnails &&
         image_upload.filesize < SiteSetting.max_image_size_kb.kilobytes &&
+        image_upload.read_attribute(:width) &&
+        image_upload.read_attribute(:height) &&
         enqueue_if_missing &&
         Discourse.redis.set(thumbnail_job_redis_key([]), 1, nx: true, ex: 1.minute)
       Jobs.enqueue(:generate_topic_thumbnails, { topic_id: id })