Take into account media_entity_pipeline_start() may fail.

[s.nawrocki: rebased onto latest tree]
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>