Refactor/Bugfix: VPS handling
- create a "dummy" VPS with ID 0 on first access in ParameterSetManager
- actually use the "dummy" VPS
- improve DecLib::m_vps handling (may still fail if multiple VPSs are in the bitstream)
Note: This fixes just some problems I noted when reviewing !1471 (merged) (but unrelated to the MR itself)
Tested with CTC.