fix ffi warning on failed to drop

[jimexist]

Which issue does this PR close?

Closes #.

Rationale for this change

fix ffi warning on failed to drop

warning: unused return value of `CString::from_raw` that must be used
   --> arrow/src/ffi.rs:135:5
    |
135 |     CString::from_raw(schema.format as *mut c_char);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default
    = note: call `drop(from_raw(ptr))` if you intend to drop the `CString`

warning: unused return value of `CString::from_raw` that must be used
   --> arrow/src/ffi.rs:137:9
    |
137 |         CString::from_raw(schema.name as *mut c_char);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: call `drop(from_raw(ptr))` if you intend to drop the `CString`

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #893 (826ba96) into master (2a0a4c3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #893   +/-   ##
=======================================
  Coverage   82.31%   82.32%           
=======================================
  Files         168      168           
  Lines       48123    48123           
=======================================
+ Hits        39614    39615    +1     
+ Misses       8509     8508    -1     

Impacted Files	Coverage Δ
arrow/src/ffi.rs	84.69% <100.00%> (ø)
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.23%)	⬇️
arrow/src/array/transform/mod.rs	85.47% <0.00%> (+0.13%)	⬆️
arrow/src/datatypes/datatype.rs	65.36% <0.00%> (+0.43%)	⬆️

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