utf8n_to_uvuni(): Fix broken malformation interactions

utf8n_to_uvuni(): Fix broken malformation interactions

All code points whose UTF-8 representations start with a byte containing
either \xFE or \xFF are considered problematic because they are not
portable.  There are many such code points that are too large to
represent on a 32 or even a 64 bit platform.  Commit
eb83ed87110e41de6a4cd4463f75df60798a9243 failed to properly catch
overflow when the input flags to this function say to warn on, but
otherwise accept FE and FF sequences.  Now overflow is checked for
unconditionally.