remove allocation in oneelement mul

[jishnub]

Removes an inadvertent allocation in the 5-el mul! with a OneElement, in the beta != 0 case

On master

julia> A = Fill(2, 4, 4);

julia> B = OneElement(1, (1,2), (4,4));

julia> @btime mul!($(zero(B)), $A, $B, 1.0, 1.0);
  87.987 ns (1 allocation: 96 bytes)

This PR

julia> @btime mul!($(zero(B)), $A, $B, 1.0, 1.0);
  20.579 ns (0 allocations: 0 bytes)

[codecov]

Codecov Report

Merging #255 (e1acc95) into master (c9d7d98) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #255   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files           5        5           
  Lines         774      774           
=======================================
  Hits          772      772           
  Misses          2        2           

Impacted Files	Coverage Δ
src/oneelement.jl	100.00% <100.00%> (ø)

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