Fix garbage DTS values on short encode sessions

[Akilan-Sivakumar]

• Encoder::Encoder() never initializes m_firstPts, m_bframeDelayTime, or m_prevReorderedPts[].
• If a stream is flushed before enough frames arrive to set m_bframeDelayTime legitimately, it retains whatever garbage was left on the heap (often leaked from a prior encoder session in long-running processes like ffmpeg), producing nonsensical DTS values.
• Fix: initialize all three fields to 0 in the constructor, so the fallback DTS calculation (reorderedPts - 0) is correct when no real B-frame reordering has occurred yet.
• Fixes Garbage DTS for short sequences #781