adaptivedemux: Check live seeking range more often

adaptivedemux: Check live seeking range more often

The live seeking range was only checked when doing actual seeks. This was
assuming that the rate would always be 1.0 (i.e. the playback would
advance in realtime, and therefore fragments would always be available
since the seeking window moves at the same rate).

With non-1.0 rates, this no longer becomes valid, and therefore we need
to check whether we are still within the live seeking range when advancing.

https://bugzilla.gnome.org/show_bug.cgi?id=783075