ozdemo: fix GCC truncation warning

[GitMensch]

../demos/ozdemo.c: In function 'main':
../demos/ozdemo.c:355:17: warning: 'strncpy' output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
                 strncpy(scrollbuf + i, message, msg_len);
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../demos/ozdemo.c:345:27: note: length computed here
             int msg_len = strlen(message);
                           ^~~~~~~~~~~~~~~

proudly presented by

gcc.exe (Rev1, Built by MSYS2 project) 8.2.0

[wmcbrine]

The code is actually working as intended here. But, I wonder if it might be possible to rewrite it in a way that's more efficient, that will also kill the warning as a side effect...

[wmcbrine]

OK, did that.

[GitMensch]

Arguably better code. Also fixes the warning and still works.