fix mockanalyzer to not use asserts [LUCENE-6127]

Currently MockAnalyzer's checks are implemented with assert. But this is just for convenience, we should not rely upon asserts being turned on: this is the whole point of the class.

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Migrated from [LUCENE-6127](https://issues.apache.org/jira/browse/LUCENE-6127) by Robert Muir (@rmuir)
Attachments: [LUCENE-6127.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6127/LUCENE-6127.patch)

Locate MockAnalyzer in the Lucene source and inspect how its checks are currently implemented. Review the attached LUCENE-6127.patch for context, then verify that the checks remain effective without relying on assertions; the issue is done when MockAnalyzer enforces them regardless of assertion settings.

Written by the indexing model from the issue text.