avcodec/dvdsubdec: reject some broken packets

If cmd_pos is broken, this would just keep accumulating packets in the
reassembly buffer, until it fails and flushes the buffer on overflow.
Since packets are usually rather small, this will take a lot of subtitle
packets. The perceived effect is that subtitles are not displayed
anymore after the faulty packet was passed to the decoder.

I'm not terribly sure about this, but on the other hand this code is
active only when fragmented packets need to be reassembled.

Fixes sample file in trac issue #4872.
