tsmux: Recheck existing pad PIDs when requesting a new pad with a random pid
Previously pads might have been requested already (e.g. in NULL state), then reset was called (e.g. because changing state) and then a new pad was requested. Resetting is re-creating the internal muxer object and as such resetting the pid counter, so the next requested pad would get the same pid as the first requested pad which then leads to collisions.
CC @meh as this is apparently some fallout from creating the proper base class.