small-fix: fix_one_variable_helper not-parallel behavior

[arnaucube]

I was using the parallel & non-parallel versions of fix_one_variable_helper, and found out that they differ. This PR adapts the non-parallel version to behave as the parallel one, since is the same behavior than in arkworks/algebra/poly/src/evaluations/multivariate/multilinear/dense.rs.

[CLAassistant]

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