lib: sbi: Fix missing '\0' when buffer szie equal 1

Fix special case: sbi_snprintf(out, out_len, ...) when out_len equal
1, The previous code will not fill the buffer with any char.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>