FIX: Prevent thumbnail gen if image too large (#10247)

It's possible through an import or other means to have images larger
than the current max allowed image size in the db.

If this happens the thumbnail generation job will keep running
indefinitely trying to download a new copy of the original but
discarding it because it is larger than the max_file_size eventually
causing this error

`Job exception: undefined method `path' for nil:NilClass`

because the newly downloaded image is now nil.

This fix stops the enqueuing of the `GenerateTopicThumbnails` job for
all images that happen to be larger than the max image size.

spec/models/topic_thumbnail_spec.rb

@@ -5,6 +5,9 @@ describe "TopicThumbnail" do
   let(:upload1) { Fabricate(:image_upload, width: 5000, height: 5000) }
   let(:topic) { Fabricate(:topic, image_upload: upload1) }
 
+  let(:upload2) { Fabricate(:image_upload, width: 5000, height: 5000, filesize: 8000) }
+  let(:topic2) { Fabricate(:topic, image_upload: upload2) }
+
   before do
     SiteSetting.create_thumbnails = true
     topic.generate_thumbnails!(extra_sizes: nil)
@@ -15,6 +18,16 @@ describe "TopicThumbnail" do
     expect(topic.topic_thumbnails.length).to eq(1)
   end
 
+  it "does not enque job if original image is too large" do
+    SiteSetting.create_thumbnails = true
+    topic2.generate_thumbnails!(extra_sizes: nil)
+
+    TopicThumbnail.ensure_consistency!
+    topic2.reload
+
+    expect(topic2.topic_thumbnails.length).to eq(0)
+  end
+
   it "cleans up deleted uploads" do
     upload1.delete