mpegvideoparse: do not set delta unit flag on unknown frame type

When encoding an image to mpeg2 video, with something like: gst-launch-1.0 encodebin name=e profile=mpegpsmux:video/mpeg,mpegversion=2,systemstream=false ! filesink location=sample.mpg filesrc num-buffers=1 blocksize=$(stat -c%s sample.png) location=sample/dts.png ! pngdec ! e. The only frame's type is set to an invalid value 0 The consequence is that mpegvideoparse sets the delta unit flag on the buffer because it is not an I frame, then decodebin3 drops this only frame because the delta unit flag is set and the decoder receives eos before it was able to receive any encoded data