Fix unitful 3-arg *

[dkarrasch]

Fixes #1291.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (189d49d) to head (8f27f1f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1499      +/-   ##
==========================================
- Coverage   93.93%   93.92%   -0.01%     
==========================================
  Files          34       34              
  Lines       15982    15983       +1     
==========================================
  Hits        15012    15012              
- Misses        970      971       +1     

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[mcabbott]

I don't see why this is related. Appears safe, given that we've already dealt with beta!=0 above.

But I'm confused about the case of nonzero beta and non-empty A,B in code just above. It seems that this code does C .*= beta, but then ignores A, B? Maybe I'm missing something. Not new to this PR. Haven't looked at tests for this.

[dkarrasch]

Yes, in that part it does C .*= beta. If iszero(alpha), then this is all that needs to be done in

C = A*B*alpha + C*beta

This change is unrelated, it just avoids "running empty loops" in the sequel.