We didn't actually check this correctly at all, but gladly overwrote the
allocated part of the output PDF, thus obviously rendering it invalid.

The parameter passed to PORT_NewArea is a default chunk size, not a maximum
anything, so it was misleading, even if not wrong as such, to pass
MAX_SIGNATURE_CONTENT_LENGTH to it. Use 10000 instead.

No need to do the overflow check twice in the Win32 case.

Change-Id: Ifa796dbb74b32e857f7184c1e8ada97ba124b020