commit b31eb901 upstream.

Setting a non-settable selection target caused BUG() to be called. The check
for valid selections only takes the selection target into account, but does
not tell whether it may be set, or only get. Fix the issue by simply
returning an error to the user.
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>