[media] media: entity: Catch unbalanced media_pipeline_stop calls

Drivers must not perform unbalanced calls to stop the entity pipeline,
however if they do they will fault in the core media code, as the
entity->pipe will be set as NULL. We handle this gracefully in the core
with a WARN for the developer.

Replace the erroneous check on zero streaming counts, with a check on
NULL pipe elements instead, as this is the symptom of unbalanced
media_pipeline_stop calls.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pincharts@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>