can: dev: make can_change_state() robust to be called with cf == NULL

In OOM situations where no skb can be allocated, can_change_state() may
be called with cf == NULL. As this function updates the state and error
statistics it's not an option to skip the call to can_change_state() in
OOM situations.

This patch makes can_change_state() robust, so that it can be called
with cf == NULL.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>