Fail blob creation if length overflows or is too large

Fail blob creation if length overflows or is too large

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.