fix compiler warning

I get:

````
src/utf8.c:645:20: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
        if (length < ab)
            ~~~~~~ ^ ~~
1 error generated.
````

with:

````
cutef8 master $ cc -v
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.2.0
Thread model: posix
cutef8 master $ 
````