Fix cat of zero-width SInt (bp #2116)

[mergify]

This is an automatic backport of pull request #2116 done by Mergify.

Cherry-pick of fd55c51 has failed:

On branch mergify/bp/1.3.x/pr-2116
Your branch is up to date with 'origin/1.3.x'.

You are currently cherry-picking commit fd55c51b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/test/scala/firrtlTests/ZeroWidthTests.scala

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/main/scala/firrtl/passes/ZeroWidth.scala

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.io/