Avoid illegal memory access when malformed data is read from the pipe

Avoid illegal memory access when malformed data is read from the pipe

- If only 1 byte is read from the interactive service client pipe, that
  evaluates to zero wide characters and subsequent check for NUL
  termination in the data buffer segfaults.
  Fix: reject clients that send less than a complete wide character.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1508520356-18277-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15657.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6f20808c8f37301c43d822f6a22d30b3587abc57)