[Libreoffice] Don't use plain "char" if you want to put negative values into it

Remember that char is either signed or unsigned, depending on
platform. Sure, char being signed is the much more common thing, but
for instance on ARM Linuxes it is unsigned. So code like:

char nDiff = bDown ? 1 : -1;
...
if( (-1 == nDiff && 0 >= nLevel) ||
     (1 == nDiff && MAXLEVEL - 1 <= nLevel))

is wrong. nDiff will never be -1.

One more reason to always build with --enable-werror.

--tml