mixclient: remove shadowed roots declaration

[yzxcj797]

Fixes #3777.

Problem

In mixing/mixclient/client.go (receiveOwnRoots wait loop), each factored-polynomial iteration begins with roots = roots[:0] on the outer roots, then declares a fresh inner roots := make([]*big.Int, 0, sesRun.mtot) that shadows it (#3777):

roots = roots[:0]                                  // resets the outer slice — uselessly
...
if len(fp.Roots) != len(a)-1 { continue }
roots := make([]*big.Int, 0, sesRun.mtot)          // shadows the outer variable
if len(fp.Roots) <= len(roots) {                   // len(roots) is always 0 → dead check
    continue
}

The len(fp.Roots) <= len(roots) guard compares against a freshly-created slice, so it is dead code; and the per-iteration reset of the outer slice does nothing.

Fix

Remove the shadowing declaration and the dead length check. The loop appends to the reset outer roots (capacity len(a)-1, exactly the number of roots a valid factored polynomial carries), and the return roots, nil at the bottom of the loop returns the same slice that was built. Behavior is unchanged — the returned values are identical — but there is one variable, one reset, and no dead branch left to mislead.

Validation

go build ./... and go vet ./... in the mixing module are clean.