lib: sbi: Fix timing of clearing tbuf

A single scan of the format char may add multiple characters to the
tbuf, causing a buffer overflow. You should check if tbuf is full in
printc so that it does not cause a buffer overflow.

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