Fix uninitialized pointer dereference in media_device_add_entity

Fix uninitialized pointer dereference in media_device_add_entity

When the entity being added has the default flag set, the function tries
to set the entity as the default entity for its type by storing the
entity pointer in the appropriate default entity field. If the entity
type isn't recognized, the default entity pointer is left uninitialized,
leading to a crash. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>