lib: sbi: Fix sbi_snprintf

printc would happily write to 'out' even when 'out_len' was zero,
potentially overflowing buffers. Rework printc to not do that and
also ensure the null byte is written at the last position when
necessary, as stated in the snprintf man page. Also, panic if
sprintf or snprintf are called with NULL output strings (except
the special case of snprintf having a NULL output string and
a zero output size, allowing it to be used to get the number of
characters that would have been written). Finally, rename a
goto label which clashed with 'out'.

Fixes: 9e8ff05c

 ("Initial commit.")
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>