Silence UBSan signed-integer-overflow warnings

[behdad]

Sometimes, like in Kerx Format6, the font can contain kerning or otherwise positioning values that are 32bit. This results in easy overflow target for fuzzing. We don't want to do anything against these, as there's no point.

As per https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html we can use __attribute__((no_sanitize("signed-integer-overflow"))) to silence these.