Security fix, CVE-2017-14491 DNS heap buffer overflow.

Fix heap overflow in DNS code. This is a potentially serious
security hole. It allows an attacker who can make DNS
requests to dnsmasq, and who controls the contents of
a domain, which is thereby queried, to overflow
(by 2 bytes) a heap buffer and either crash, or
even take control of, dnsmasq.

src/option.c

@@ -1415,7 +1415,7 @@ static int parse_dhcp_opt(char *errstr, char *arg, int flags)
 		    }
 		  
 		  p = newp;
-		  end = do_rfc1035_name(p + len, dom);
+		  end = do_rfc1035_name(p + len, dom, NULL);
 		  *end++ = 0;
 		  len = end - p;
 		  free(dom);