fix handling of invalid UTF8 sequences

external data (and in bad cases our own) can contain invalid byte
sequences in UTF8 strings. A prominent example are file names.
There was a 1-off bug in calculating the allowed length for multibyte
chars, and the iterator was a bit too greedy when stumbling upon
invalid sequences, returning a single "invalid" char for a sequence
up to the point where it became invalid in calculation. Now, we present
one invalid char for the first byte of that sequence and then check
for a valid char starting with the next byte.