Allocate/DeAllocate video buffers using av_malloc

Also ensure frame.qscale_table is freed with av_free as it was allocated with av_malloc by FFmpeg
Fix memory leak

Coverity CID 1224482
Coverity CID 1224483

If pginfo was null, the line above would have crashed. pginfo is never null
Ensure class members are initialised in constructor

coverity CID 1224484