Problem:  buffer-overflow in trunc_string()
Solution: Add NULL at end of buffer

Currently trunc_string() assumes that when the string is too long,
buf[e-1] will always be writeable. But that assumption may not always be
true. The condition currently looks like this

    else if (e + 3 < buflen)
    [...]
    else
    {
	// can't fit in the "...", just truncate it
	buf[e - 1] = NUL;
    }

but this means, we may run into the last else clause with e still being
larger than buflen. So a buffer overflow occurs.

So instead of using `buf[e - 1]`, let's just always
truncate at `buf[buflen - 1]` which should always be writable.

https://github.com/vim/vim/commit/3bd7fa12e146c6051490d048a4acbfba974eeb04

vim-patch:9.0.2004: Missing test file

Problem:  Missing test file
Solution: git-add the file to the repo

closes: vim/vim#13305

https://github.com/vim/vim/commit/d4afbdd0715c722cfc73d3a8ab9e578667615faa

Co-authored-by: Christian Brabandt <cb@256bit.org>