win_redr_custom: pad `buf` with NUL / fix #858

Posting this PR to see if it fixes the problem #858.

https://github.com/neovim/neovim/issues/858#issuecomment-46318842

> Quick guess: vim_strncpy fills buf with NUL to the end. STRLCPY does not "clear" buf, so 
for windows with small maxwidth the following while loop won't set enough fillchars to "clear" buf.

I still don't see how STRLCPY causes a problem here...