CVE-2015-7547: getaddrinfo() stack-based buffer overflow (Bug 18665).

CVE-2015-7547: getaddrinfo() stack-based buffer overflow (Bug 18665).

* A stack-based buffer overflow was found in libresolv when invoked from
  libnss_dns, allowing specially crafted DNS responses to seize control
  of execution flow in the DNS client.  The buffer overflow occurs in
  the functions send_dg (send datagram) and send_vc (send TCP) for the
  NSS module libnss_dns.so.2 when calling getaddrinfo with AF_UNSPEC
  family.  The use of AF_UNSPEC triggers the low-level resolver code to
  send out two parallel queries for A and AAAA.  A mismanagement of the
  buffers used for those queries could result in the response of a query
  writing beyond the alloca allocated buffer created by
  _nss_dns_gethostbyname4_r.  Buffer management is simplified to remove
  the overflow.  Thanks to the Google Security Team and Red Hat for
  reporting the security impact of this issue, and Robert Holiday of
  Ciena for reporting the related bug 18665. (CVE-2015-7547)

See also:
https://sourceware.org/ml/libc-alpha/2016-02/msg00416.html
https://sourceware.org/ml/libc-alpha/2016-02/msg00418.html

(cherry pick from commit e9db92d3acfe1822d56d11abcea5bfc4c41cf6ca)