Fix shadowed variable in getRandomPolynomial to correctly disable blinding (PLONK bn254)

[viktorking7]

Description

• Summary: Replace a shadowed assignment that prevented creating a zero polynomial when n == -1.
• Problem: In getRandomPolynomial, a was redeclared with a := under n == -1, leaving the outer slice nil and failing to cancel blinding as intended.
• Fix: Change a := make(... to a = make(... so the outer variable is set.
• Impact: Ensures correct “cancel the blinding” behavior.

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Note

Fixes a shadowed variable so getRandomPolynomial(-1) returns a zero polynomial, correctly canceling blinding.

• PLONK bn254 backend:
  • In backend/plonk/bn254/prove.go, update getRandomPolynomial to assign to existing a instead of redeclaring (a = make(...)), ensuring n == -1 produces a zero polynomial to cancel blinding.

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