powerpc/powernv: Add a null pointer check in opal_event_init()

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.

Fixes: 2717a33d ("powerpc/opal-irqchip: Use interrupt names if present")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231127030755.1546750-1-chentao@kylinos.cn