lib: sbi: Fix printc

Because *out needs to reserve a byte to hold '\0', no more characters
should be added to the buffer when *out has one byte left, and the
buffer size *out_len should not be modified. this patch prevents
the correction of *out_len when *out_len is 1.

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