video: fix accel when setting is disabled

If Ring was compiled with acceleration, but is disabled in dring.yml,
the call would crash. This checks if the accel_ object isn't null
before checking if it has failed.

Change-Id: I098f573bd17271824fcfbdb60d783af085ef19be
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>