DVDDemuxFFmpeg: Fix calculation when increasing buffer

This was here since: https://github.com/xbmc/xbmc/commit/ee6dd202

@FernetMenta what do you think? @elupus: What was the idea behind this? Or rephrase: Why did this fix the homerun issue?

Problem here is, that this wants to workaround the case when FFMPEG_FILE_BUFFER_SIZE is smaller than max_packet_size, but if that is the case the division will result in plain 0 as both are integers. Therefore the max_packet_size is set to 0.

Thx @t-nelson for spotting.

this code basically does this:

```
if (m_ioContext->max_packet_size > FFMPEG_FILE_BUFFER_SIZE)
  (m_ioContext->max_packet_size = FFMPEG_FILE_BUFFER_SIZE;
```

You should not introduce floats and rounding errors. this may end up in some odd size which you don't want to request from the input stream.

again my question: what input stream reports such a big block size?