MIPS: OCTEON: irq: Fix potential NULL pointer dereference

There is a potential NULL pointer dereference in case kzalloc()
fails and returns NULL.

Fix this by adding a NULL check on *cd*

This bug was detected with the help of Coccinelle.

Fixes: 64b139f9 ("MIPS: OCTEON: irq: add CIB and other fixes")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>