Throw an exception if examined < consumed and reworked lastExamined (dotnet/corefx...

Throw an exception if examined < consumed and reworked lastExamined (dotnet/corefx#35879)

- Store a single absolute position instead of the segment
- Fixed some typos and added tests for null segments
- Added more tests

Commit migrated from https://github.com/dotnet/corefx/commit/203bc37eddbae8fb14531d305554a662c0428279