Fix bug in zero-width memory removal (bp #2153)

[mergify]

This is an automatic backport of pull request #2153 done by Mergify.

Cherry-pick of 67ce97a has failed:

On branch mergify/bp/1.3.x/pr-2153
Your branch is up to date with 'origin/1.3.x'.

You are currently cherry-picking commit 67ce97a1.
  (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/

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