Stop using spelling "nonexistant".

Noah Misch <noah@leadboat.com>

Commit: f2c71cb71fe186dcf8ebec967f98d1c7c4777943
Author: Noah Misch <noah@leadboat.com>
Date: 2019-06-08T17:12:26Z
Releases: 12.0
Stop using spelling "nonexistant".

The documentation used "nonexistent" exclusively, and the source tree
used it three times as often as "nonexistant".

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