Fix some bugs in networking

* netdb: Fix boundary conditions in dns_recv_response()

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* Fix typos in include/nuttx/net/dns.h

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

Approved-by: Gregory Nutt <gnutt@nuttx.org>