Fix an off-by-one error in FcLangSetIndex()

This commit fixes a bug that can be reproduced like this:
  - remove all languages starting with 'a' in fc-lang/ (in
    ORTH's definition);
  - rebuild fontconfig with this change (-> new fc-lang/fclang.h);
  - create an FcLangSet 'ls1' that contains at least the first language
    from fcLangCharSets (i.e., the first *remaining* in lexicographic
    order); let's assume it is "ba" for the sake of this description;
  - create an FcLangSet 'ls2' that only contains the language "aa" (any
    language starting with 'a' should work as well);
  - check the return value of FcLangSetContains(ls1, ls2);

The expected return value is FcFalse, however it is FcTrue if you use
the code before this commit.

What happens is that FcLangSetIndex() returns 0, because this is the
index of the first slot after the not-found language "aa" in
fcLangCharSets (since we removed all languages starting with 'a').
However, this index happens to be non-negative, therefore
FcLangSetContainsLang() mistakenly infers that the language "aa" was
found in fcLangCharSets, and thus calls FcLangSetBitGet(ls1, 0), which
returns FcTrue since we've put the first remaining language "ba" in the
'ls1' language set.

The "return -low;" statement previously in FcLangSetIndex() was
inconsistent with the final return statement. "return -(low+1);" fixes
this inconsistency as well as the incorrect behavior described above.
1 file changed