CompressingTermVectors should put checkDoc and checkPosition within assert [LUCENE-6011]

CompressingTermVectorsReader.TVDocsEnum calls checkPosition() and checkDoc() but not from within asserts. And nextPosition() has some leading checks, also not within asserts. I believe these should all be within asserts. This is low-level code that can get called a ton of times, particularly via TokenSources.getTokenStream.

I'd be happy to create a patch there is preliminary agreement.

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Migrated from [LUCENE-6011](https://issues.apache.org/jira/browse/LUCENE-6011) by David Smiley (@dsmiley)

Start at CompressingTermVectorsReader.TVDocsEnum and inspect checkPosition(), checkDoc(), and nextPosition(), especially their leading checks. Review how TokenSources.getTokenStream reaches this code, then confirm the relevant checks are evaluated only through assertions without changing behavior when assertions are disabled.

Written by the indexing model from the issue text.