Fixed compile warning due to default cast

[fawdlstty]

Fixed compile warning due to default cast

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov-io]

Codecov Report

Merging #2201 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2201   +/-   ##
=======================================
  Coverage   86.47%   86.47%           
=======================================
  Files         511      511           
  Lines       83223    83223           
=======================================
  Hits        71971    71971           
  Misses      11252    11252           

Impacted Files	Coverage Δ
src/layer/roialign.cpp	92.79% <100.00%> (ø)
src/layer/x86/roialign_x86.cpp	87.07% <100.00%> (ø)
src/mat_pixel_affine.cpp	96.33% <100.00%> (ø)

---

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 bc24712...195a4dd. Read the comment docs.

[lgtm-com]

This pull request fixes 2 alerts when merging 195a4dd into bc24712 - view on LGTM.com

fixed alerts:

• 2 for Declaration hides parameter

[nihui]

Thanks for your contribution !