basesink: Make sure we never drop the preroll buffer
basesink: Make sure we never drop the preroll buffer

This is cosmetic as 'late' should never be set during preroll (in pause).
Though code may evolve in the future, so this is good for preventing
potential bugs.

https://bugzilla.gnome.org/show_bug.cgi?id=772468

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer