Thumbnail creation fails for non-default PDF layer

[conorom]

Using Layer 1, say, versus 0/default as documented here.
So using 1 means use the 2nd page as the thumbnail, which often makes sense because the first/title page is generally mostly blank. But this doesn't seem to work as intended.

As far as I can tell, it's a matter of not using the layer any more in the mogrify command after a (presumed?) successful flatten operation. If it's been flattened then you're left with only layer [0] so adding a [1] to the mogrify command makes no sense any more. As you can't see the mogrify error in the server log below, this is what it looks like (where blah.pdf is a multi-page PDF to begin with):

mogrify -flatten blah.pdf[1]
mogrify -resize 338x493 blah.pdf[1]

Requested FirstPage is greater than the number of pages in the file: 1
   No pages will be processed (FirstPage > LastPage).

Server log:

 [2017-11-30T22:50:14.952965 #16824] DEBUG -- : [ActiveJob] [CreateDerivativesJob] [8f197cbe-dfe2-4258-8221-ed2d1fe74203] [34.45s] identify /path/to/app/tmp/mini_magick20171130-16824-nkwppn.pdf
D, [2017-11-30T22:50:21.287374 #16824] DEBUG -- : [ActiveJob] [CreateDerivativesJob] [8f197cbe-dfe2-4258-8221-ed2d1fe74203] [6.33s] mogrify -flatten /path/to/app/tmp/mini_magick20171130-16824-nkwppn.pdf[1]
D, [2017-11-30T22:50:21.467658 #16824] DEBUG -- : [ActiveJob] [CreateDerivativesJob] [8f197cbe-dfe2-4258-8221-ed2d1fe74203] [0.18s] mogrify -resize 338x493 /path/to/app/tmp/mini_magick20171130-16824-nkwppn.pdf[1]
E, [2017-11-30T22:50:21.468698 #16824] ERROR -- : [ActiveJob] [CreateDerivativesJob] [8f197cbe-dfe2-4258-8221-ed2d1fe74203] Error performing CreateDerivativesJob (Job ID: 8f197cbe-dfe2-4258-8221-ed2d1fe74203) from Resque(ingest) in 84266.43ms: MiniMagick::Error (`mogrify -resize 338x493 /path/to/app/tmp/mini_magick20171130-16824-nkwppn.pdf[1]` failed with error:
):
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/mini_magick-4.8.0/lib/mini_magick/shell.rb:17:in `run'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/mini_magick-4.8.0/lib/mini_magick/tool.rb:92:in `call'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/mini_magick-4.8.0/lib/mini_magick/tool.rb:40:in `new'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/mini_magick-4.8.0/lib/mini_magick/image.rb:564:in `mogrify'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/mini_magick-4.8.0/lib/mini_magick/image.rb:455:in `method_missing'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/hydra-derivatives-3.3.2/lib/hydra/derivatives/processors/image.rb:25:in `block in create_resized_image'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/hydra-derivatives-3.3.2/lib/hydra/derivatives/processors/image.rb:32:in `create_image'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/hydra-derivatives-3.3.2/lib/hydra/derivatives/processors/image.rb:22:in `create_resized_image'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/hydra-derivatives-3.3.2/lib/hydra/derivatives/processors/image.rb:8:in `process'
/path/to/shared/vendor/bundle/ruby/2.4.0/gems/hydra-derivatives-3.3.2/lib/hydra/derivatives/runners/runner.rb:30:in `block (2 levels) in create'