libndp: fix [cppcheck] Undefined behavior: Variable 'buf' is used as parameter and destination in s[n]printf()

cppcheck --enable=all --inconclusive --std=posix .

ndp_msg_opt_dnssl_domain():
			if (dom_len > len)
				return NULL;

			if (strlen(buf))
---->				sprintf(buf, "%s.", buf);
			buf[strlen(buf) + dom_len] = '\0';
			memcpy(buf + strlen(buf), ptr, dom_len);

So just use strcat instead.

Reported-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>