lib: sbi: sse_event_get() may return NULL

sse_event_get() may return NULL. We should not dereference the return value
in sbi_sse_exit() without checking.

Fixes: c8cdf01d

 ("lib: sbi: Add support for Supervisor Software Events extension")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Anup Patel <anup@brainfault.org>