Workaround nightly miri error in try_from_trusted_len_iter

[jhorstmann]

Which issue does this PR close?

Closes #1496.

Rationale for this change

I can't really explain why miri complained about this code, but the Result here is the only difference to several other usages of to_byte_slice. Extracting the result handling to a separate line seems to fix the issue.

[codecov-commenter]

Codecov Report

Merging #1497 (ebb23d0) into master (c5442cf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head ebb23d0 differs from pull request most recent head 161a8eb. Consider uploading reports for the commit 161a8eb to get more accurate results

@@            Coverage Diff             @@
##           master    #1497      +/-   ##
==========================================
- Coverage   82.68%   82.68%   -0.01%     
==========================================
  Files         188      188              
  Lines       54361    54362       +1     
==========================================
- Hits        44951    44947       -4     
- Misses       9410     9415       +5     

Impacted Files	Coverage Δ
arrow/src/buffer/mutable.rs	90.53% <100.00%> (+0.02%)	⬆️
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.46%)	⬇️
arrow/src/datatypes/datatype.rs	66.40% <0.00%> (-0.40%)	⬇️
arrow/src/array/transform/mod.rs	86.35% <0.00%> (-0.23%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c5442cf...161a8eb. Read the comment docs.

[alamb]

If it makes MIRI happy it makes me happy 😆

[alamb]

Thanks @jhorstmann