Fixes for several mythtranscode Coverity IDs

- Initialize m_last_audiotime in AudioReencodeBuffer ctor. ID 746889.
- Initialize totalFrames in Cutter ctor. ID 1025912.
- Check return value of preview image rename() call in main.cpp.  ID 746754.
- Execution can not reach "NULL" in mpeg2fix.cpp. ID 746800.