refactor my_strupr

refactor my_strupr

Refactor my_strupr such that it will not check and possibly alter bytes
after the string's null terminator for strings of length 0.

Signed-off-by: Guido Vranken <guidovranken@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <CAO5O-ELXwXgGtRFou=FY1TAhW4W4BHPZF4=rqSY877njtXNMJg@mail.gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14756.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 69162924de3600bfe8ae9708a1d6e3f4515ef995)