lib: sbi: Fix possible buffer overrun in counter validation

The active_events array is accessed with counter ID passed from the supervisor
software before the counter ID bound check. This may cause a buffer overrun
if a supervisor passes an invalid counter ID.

Fix this by moving the access part after the bound check.

Reported-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>