Fix asserts that are failing on .NET Standard 2.0

There are a few asserts that fail on .NET Standard 2.0. They have been conditionally compiled out of the code due to the fact that on .NET Core 2.1 they cause the test runner to fatally crash, however these are a sign that something could be seriously wrong with the codecs (possibly causing index corruption).

Here are the known failures:

• Lucene3xTermVectorsReader
• Lucene40TermVectorsReader
• BlockTreeTermsWriter