tiny-printf: emit \0 as %c

tiny-printf: emit \0 as %c

The current code has a problematic corner case with formar "%c" and
0 as parameter. The proper zero byte is being emitted into digit buffer
but the final copy into outstr expects null-terminated string and doesn't
copy the required \0 byte. This has lead to malformed TFTP packets, refer
to tftp_send() which relies on %c to generate multiple zero-terminated
strings in one buffer.

Introduce a variable to force the copy of one character in this case.
The new behaviour is consistent with non-tiny implementation.

Reported-by: Chintan Vankar <c-vankar@ti.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>