Fix uses of unsafe arithmatics

[lgoettgens]

one should not rely on modification of the first arg, but instead take the return value

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.00%. Comparing base (86996ee) to head (460e129).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Matrix-Strassen.jl	92.00%	2 Missing ⚠️
src/Matrix.jl	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1790   +/-   ##
=======================================
  Coverage   87.99%   88.00%           
=======================================
  Files         117      117           
  Lines       30037    30037           
=======================================
+ Hits        26432    26434    +2     
+ Misses       3605     3603    -2     

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