Avoid mutation on user x

[ChrisRackauckas]

Fixes #265

[codecov]

Codecov Report

Attention: 31 lines in your changes are missing coverage. Please review.

Comparison is base (dc28eb2) 85.93% compared to head (54ce5d5) 41.77%.

Files	Patch %	Lines
src/differentiation/jaches_products.jl	0.00%	17 Missing ⚠️
src/differentiation/vecjac_products.jl	0.00%	9 Missing ⚠️
ext/SparseDiffToolsZygoteExt.jl	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #280       +/-   ##
===========================================
- Coverage   85.93%   41.77%   -44.16%     
===========================================
  Files          22       22               
  Lines        1358     1338       -20     
===========================================
- Hits         1167      559      -608     
- Misses        191      779      +588     

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