Strings that include Latin1 encoding or corrupt UTF8 byte sequences
could read past the buffer bounds (stepping over the null terminator).
Resolve by passing in the string length.
Other changes to support non-UTF8 byte sequences:
- BLI_str_utf8_offset_{to/from}_index were accumulating
the UTF8 offset without accounting for non-UTF8 characters
which could cause a buffer underflow or enter an eternal loop.
- BLI_str_utf8_offset_to_index would read past the buffer bounds if the
offset passed in if it was in the middle of a UTF8 byte sequence.
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