net: lwip: dhcp: Do not write past end of buffer

net: lwip: dhcp: Do not write past end of buffer

sprintf will write a trailing \0 at the end of the string so when
writing into a buffer, that buffer must be sized to allow for that
trailing zero. In the DHCP code when the index is a number needing two
digits to express the index would use up the two \0 bytes in the buffer
and the trailing \0 from sprintf would be beyond the end of the
allocation. Fix this by adding a third \0 in the buffer.

This was found by code inspection when looking for an issue reported by
Michal Simek, but I do not have the hardware to reproduce, so cannot
confirm if this addresses that issue or not.

Fixes: 98ad145db61a ("net: lwip: add DHCP support and dhcp commmand")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@arm.com>