gst/gstpipeline.c: Don't ref a NULL clock in _provide_clock_func().

gst/gstpipeline.c: Don't ref a NULL clock in _provide_clock_func().

Original commit message from CVS:
* gst/gstpipeline.c: (gst_pipeline_change_state),
(gst_pipeline_provide_clock_func), (gst_pipeline_set_delay):
Don't ref a NULL clock in _provide_clock_func().
Don't allow an INVALID delay.
Don't try to calculate base_time with an invalid start_time.
Also distribute and notify a NULL clock when it was selected.
* tools/gst-launch.c: (event_loop):
Don't crash when a NULL clock was selected in the pipeline.