Minor, make MSVC happy by fixing memory safety warnings

These probably are [false] alarms of MSVC but this is fix them. I don't know if this even worth to do and probably [{{0}}](http://stackoverflow.com/a/5434963/1414809) is better to be used on hb-ot-shape-complex-arabic-fallback.hh but actually I wanted to get more familiar with harfbuzz development. It compiles with cygwin64 GCC and MSVC and make check (# TOTAL: 8 # PASS:  8)