GH-50355: [C++][Gandiva] fix out-of-bounds read in utf8_length_ignore_invalid

[Arawoof06]

Rationale for this change

utf8_length_ignore_invalid extends char_len while scanning continuation bytes and never rechecks the buffer end, so an input ending in a truncated multi-byte utf8 sequence (a 0xF0 lead byte followed by non-continuation bytes) reads past data_len. It is reached from untrusted string data through lpad/rpad, which count the input glyphs before padding. Reproduced against a verbatim copy of the function under AddressSanitizer with the 4-byte input {0xF0, 'a', 'a', 'a'} in an exactly-sized heap buffer, giving heap-buffer-overflow READ ... 0 bytes after 4-byte region.

What changes are included in this PR?

Bound the continuation-byte scan with i + j < data_len, the same guard the sibling helpers (utf8_length, reverse_utf8, utf8_byte_pos) already use. Valid input is unaffected because a well-formed glyph always satisfies i + char_len <= data_len.

Are these changes tested?

Yes. Added TestStringOps.TestPadMalformedUtf8NoOverread, which runs lpad/rpad on the truncated multi-byte input placed in an exactly-sized heap buffer so the over-read trips ASAN; the existing pad tests still pass.

Are there any user-facing changes?

No.

This PR contains a "Critical Fix". It fixes an out-of-bounds read in the Gandiva utf8 length helper reachable from lpad/rpad on crafted string data.

• GitHub Issue: [C++][Gandiva] Out-of-bounds read in utf8_length_ignore_invalid on truncated multi-byte input #50355

[github-actions]

⚠️ GitHub issue #50355 has been automatically assigned in GitHub to PR creator.