vm: clone struct item on SETITEM, close #972

[roman-khimov]

It's the same as with APPEND and VALUES.

[codecov]

Codecov Report

Merging #975 into master-2.x will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           master-2.x     #975      +/-   ##
==============================================
- Coverage       68.00%   67.99%   -0.02%     
==============================================
  Files             145      145              
  Lines           14312    14313       +1     
==============================================
- Hits             9733     9732       -1     
- Misses           4119     4121       +2     
  Partials          460      460              

Impacted Files	Coverage Δ
pkg/vm/vm.go	86.66% <100.00%> (+0.01%)	⬆️
pkg/rpc/client/wsclient.go	84.35% <0.00%> (-1.37%)	⬇️

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