Update the buffering state before stalling for more data

Update the buffering state before stalling for more data

In some cases the wait for more data was happening without updating
the buffering state, meaning the API user would not be able to notice
it should pause the pipeline and update UI to indicate that is the
case, the video would likely stutter instead.

https://bugzilla.gnome.org/show_bug.cgi?id=707648