1.0.32.17: make the utf-8 external format more robust

1.0.32.17: make the utf-8 external format more robust

Detect all malformed sequences, including attempts to decode or encode
Unicode surrogate codepoints (disallowed by the Unicode definition of
UTF-8).  Some error tests change behaviour, and some (unexported)
condition classes are not triggered under the same circumstances any
more.

Also, handle null-termination on a successful conversion of an empty range of
a nil array.