libs/gst/base/gstbasesink.c: Protect eos and have_preroll with the OBJECT lock so we don't need to take the PREROLL l...

Original commit message from CVS:
* libs/gst/base/gstbasesink.c: (gst_base_sink_preroll_queue_flush),
(gst_base_sink_wait_preroll), (gst_base_sink_needs_preroll),
(gst_base_sink_query):
Protect eos and have_preroll with the OBJECT lock so we don't need to
take the PREROLL lock when querying the latency. Fixes #473846.