can: ctucanfd: handle skb allocation failure

can: ctucanfd: handle skb allocation failure

If skb allocation fails, the pointer to struct can_frame is NULL. This
is actually handled everywhere inside ctucan_err_interrupt() except for
the only place.

Add the missed NULL check.

Found by Linux Verification Center (linuxtesting.org) with SVACE static
analysis tool.

Fixes: 2dcb8e8782d8 ("can: ctucanfd: add support for CTU CAN FD open-source IP core - bus independent part.")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/20250114152138.139580-1-pchelkin@ispras.ru
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>