GH-50707: [C++][Gandiva] fix out-of-bounds read in mask utf8proc length

[Arawoof06]

Rationale for this change

The Gandiva mask stubs walk a utf8 string calling utf8proc_iterate(data + bytes_read, data_len, ...), passing the total data_len as the remaining byte count even though the pointer has already advanced by bytes_read. utf8proc_iterate reads up to strlen bytes, so a value ending in a truncated multi-byte glyph (for example a lead byte 0xF0 as the final byte of an exactly sized value buffer) makes it read continuation bytes past data + data_len. This is reachable from mask(), mask_first_n() and mask_last_n() on untrusted string columns. gdv_mask_last_n_utf8_int32 happens to be shielded by its utf8proc_decompose pre-pass, but gdv_mask_first_n_utf8_int32 and mask_utf8_utf8_utf8_utf8 have no such guard and over-read.

What changes are included in this PR?

Pass data_len - bytes_read (the real remaining length) at all four utf8proc_iterate call sites so utf8proc reports the truncated glyph instead of reading past the buffer. mask_utf8_utf8_utf8_utf8 also lacked the char_len < 0 check that the other two paths already have, so I added it there to reject the now-detected invalid input rather than advancing by a negative length.

Are these changes tested?

Yes. TestMaskTruncatedUtf8NoOverread feeds a value whose reported length stops one byte short of a complete euro sign; before the change both functions consumed the out-of-range byte and returned a masked result, after it they report the truncated input. The existing mask tests still pass.

Are there any user-facing changes?

No change for valid utf8. A value that ends in a truncated multi-byte glyph now surfaces an invalid-utf8 error instead of being silently masked using bytes past its end.

This PR contains a "Critical Fix". The wrong length argument lets utf8proc_iterate read past the end of an exactly sized input buffer.

• GitHub Issue: [C++][Gandiva] Out-of-bounds read in mask functions from wrong utf8proc_iterate length #50707

[github-actions]

⚠️ GitHub issue #50707 has been automatically assigned in GitHub to PR creator.