Return NULL if GetAdaptersInfo fails

Return NULL if GetAdaptersInfo fails

- Currently a pointer to potentially uninitialized IP_ADAPTER_INFO
  struct is returned on error causing ill-defined behaviour.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Simon Rozman <simon@rozman.si>
Message-Id: <1514952123-26616-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16139.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>