Fuzzer issue #5984 no.43. Substring generating an invalid string

[LindsayWray]

Addresses fuzzer issue #5984 no.43

Problem query:

SELECT substring(decode('u\xD5\x8D1'::BLOB)::VARCHAR, -1);
——
Assertion `utf_type != UnicodeType::INVALID

The issue seemed to be when calling substring with a string containing unicode character(s) and a negative offset.
A negative offset will start indexing from the end of the string and could cut of a unicode character midway when iterating through the string.

[Mytherin]

Thanks! LGTM