gst-launch: go back down to GST_STATE_NULL in one step.

gst-launch: go back down to GST_STATE_NULL in one step.

Going through each state on the way back down to GST_STATE_NULL
can cause deadlocks, for example:

gst-launch-1.0 audiotestsrc ! valve drop=true ! autoaudiosink

ctrl + C

Hangs forever when going to PAUSED, because the "final" state is
ASYNC, and the sink blocks waiting for a preroll buffer.

Going straight to NULL addresses this issue, and also helps
making teardown faster when piping sparse streams to a
sync sink.