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.