movenc: take packet dts shifting into mention in check_pkt

This FFmpeg-specific "fuzzer fix" was never perfect, but now it
stopped encoding of actual content with a big enough DTS shift.
This returns the function to its original state of results
before negative CTS offsets were added.

I remember dealing with this function before, but somehow had
forgotten about it during VDD. The test cases not tripping this
over also didn't help.
