Fix missing check for return value of malloc'd buffer

Fix missing check for return value of malloc'd buffer

- Use utf8to16 from common.c for utf8 to wide conversion and
  check its return value

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1508094315-466-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15641.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f3d389a2d2b87aeb649bfdccd596f485346a32c7)