fix: undefined behavior in schema ffi

[roee88]

Closes #580

Rationale for this change

The undefined behavior that was fixed in #323 was re-introduced in #439.
This PR attempts to re-introduce the same fix.

[alamb]

FWIW when I tested this change locally with MIRI it passed the tests that were previously failing

alamb@MacBook-Pro arrow-rs % git cherry-pick 7e5f4d1285baacfe56bbf4026cea6bb2da873bb1
git cherry-pick 7e5f4d1285baacfe56bbf4026cea6bb2da873bb1
Auto-merging arrow/src/ffi.rs
[alamb/validate_output_of_miri 1d075e2e2] fix: undefined behavior in schema ffi
 Author: roee88 <roee88@gmail.com>
 Date: Wed Jul 21 15:33:51 2021 +0300
 1 file changed, 6 insertions(+), 3 deletions(-)
alamb@MacBook-Pro arrow-rs % MIRIFLAGS="-Zmiri-disable-isolation" cargo +nightly miri test -p arrow -- --ignored ffi
MIRIFLAGS="-Zmiri-disable-isolation" cargo +nightly miri test -p arrow -- --ignored ffi
WARNING: Ignoring `RUSTC_WRAPPER` environment variable, Miri does not support wrapping.
   Compiling arrow v6.0.0-SNAPSHOT (/Users/alamb/Software/arrow-rs/arrow)
    Finished test [unoptimized + debuginfo] target(s) in 3.04s
     Running unittests (target/miri/x86_64-apple-darwin/debug/deps/arrow-111eb02f3e835a62)

running 6 tests
test array::ffi::tests::test_struct ... ok
test datatypes::ffi::tests::test_field ... ok
test datatypes::ffi::tests::test_schema ... ok
test datatypes::ffi::tests::test_type ... ok
test ffi::tests::test_large_list ... ok
test ffi::tests::test_list ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 760 filtered out

   Doc-tests arrow

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 103 filtered out; finished in 0.01s

[alamb]

If it passes MIRI looks good to me 👍

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.45%. Comparing base (e3d4c6a) to head (7e5f4d1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #582      +/-   ##
==========================================
- Coverage   82.46%   82.45%   -0.01%     
==========================================
  Files         167      167              
  Lines       46205    46205              
==========================================
- Hits        38101    38100       -1     
- Misses       8104     8105       +1     

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[alamb]

https://github.com/apache/arrow-rs/pull/582/checks?check_run_id=3123911024

Shows:

test result: ok. 89 passed; 0 failed; 0 ignored; 0 measured; 14 filtered out; finished in 12.50s

👍

[alamb]

Thank you so much @roee88