Fail if blob start plus length overflows; or if blob length
is greater than 2GB. It takes a while for fonts to get to that
size. In the mean time, it protects against bugs like this:
http://www.icu-project.org/trac/ticket/11450
Also avoids some weird issues with 32bit vs 64bit systems
as we accept length as unsigned int. As such, a length of
-1 will cause overflow on 32bit machines, but happily
accepted on a 64bit machine. Avoid that.
