fix out-of-bounds reads in __dns_parse

fix out-of-bounds reads in __dns_parse

There are several issues with range checks in this function:

* The question section parsing loop can read up to two out-of-bounds
  bytes before doing the range check and bailing out.

* The answer section parsing loop, in addition to the same issue as
  above, uses the wrong length in the range check that doesn't prevent
  OOB reads when computing len later.

* The len range check before calling the callback is off by 10. Also,
  p+len can overflow in a (probably theoretical) case when p is within
  2^16 from UINTPTR_MAX.

Because __dns_parse is used only with stack-allocated buffers, such
small overreads can't result in a segfault. The first two also don't
affect the function result, but the last one may result in getaddrinfo
incorrectly succeeding and returning up to 10 bytes past the
response buffer as a part of the IP address, and in (canon) name
returned by getaddrinfo/getnameinfo being affected by memory past the
response buffer (because dn_expand might interpret it as a pointer).