Fix potential race condition when frame field information SEI arrives before picture timing SEI

Picture Timing SEI is intentionally not used in parsing of frame field information SEI.

In the existing code retrieving the picture timing SEI from HRD could return a nullptr, when the SEI has
not arrived yet. The pointer is dereferenced without checking and passed into frame field info SEI parsing
function and used for a consistency check.

This refactoring moves the check after parsing of all SEI messages.