Remove two potential warnings for VisualStudio

[TianZerL]

There are two warnings about data conversion(size_t to int) in mat.h when compiling on VisualStudio, this PR fixed them.

[tencent-adm]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #3188 (0e9b245) into master (5a35c2b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3188   +/-   ##
=======================================
  Coverage   90.35%   90.35%           
=======================================
  Files         511      511           
  Lines      136455   136455           
=======================================
  Hits       123300   123300           
  Misses      13155    13155           

Impacted Files	Coverage Δ
src/mat.h	87.52% <100.00%> (ø)

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[nihui]

Thanks for your contribution !