fix arithmetic bug

[jeffry1829]

fix #363

[codecov]

Codecov Report

Attention: 45 lines in your changes are missing coverage. Please review.

Comparison is base (5e4ebf5) 17.83% compared to head (f3efa90) 17.81%.
Report is 5 commits behind head on dev-master.

Files	Patch %	Lines
src/linalg/Div.cpp	7.14%	8 Missing and 5 partials ⚠️
src/linalg/Sub.cpp	7.14%	9 Missing and 4 partials ⚠️
src/linalg/Add.cpp	25.00%	4 Missing and 5 partials ⚠️
src/linalg/Mul.cpp	25.00%	4 Missing and 5 partials ⚠️
src/ncon.cpp	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           dev-master     #368      +/-   ##
==============================================
- Coverage       17.83%   17.81%   -0.03%     
==============================================
  Files             220      220              
  Lines           48726    48761      +35     
  Branches        19923    19955      +32     
==============================================
- Hits             8690     8686       -4     
- Misses          35841    35871      +30     
- Partials         4195     4204       +9     

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[kaihsin]

make corresponding change for Add and Mul