mpegpsdemux: Confused when receiving segment updates from upstream

Submitted by Guillaume Desmottes @gdesmott

Link to original bug (#734036)

Description

I'm working on implementing DLNA time based fast forward. The idea is to
implement fast forward in dlnasrc by requesting smaller parts of the file to
the HTTP server. Dlnasrc then sends segment updates for each fragment of the
file to play (for example, if rate=2 and we use chunk of 1 second we could have
something like [00:00, 00:01] then [00:02, 00:03] then [00:04; 00:05]).

This works fine with mpegpsdemux when doing fast forward but not when going in reverse to do fast backward (rate=-2 for example). It starts playing the requested chunks fine but at some point it just stops.