Fix uninitialized work->x in daqp_check_unconstrained when no linear term

[Copilot]

daqp_check_unconstrained uses work->xold (malloc'd, uninitialized) as scratch space by swapping it into work->x, then skips writing to it when work->v == NULL (no f term). The subsequent feasibility check reads garbage, causing the unconstrained shortcut to fire incorrectly—or not at all—for pure quadratic problems (min 0.5 x'Hx, no linear term).

When the shortcut fires incorrectly, sing_ind is set to DAQP_UNCONSTRAINED_OPTIMAL and the extracted primal solution is garbage from uninitialized memory. This is the root cause of the erratic minrep behaviour reported after the unconstrained-feasibility shortcut was introduced.

Changes

• src/utils.c — In daqp_check_unconstrained, add an else branch covering work->v == NULL. The unconstrained optimum of min 0.5 x'Hx is always x = 0, so zero the scratch buffer before the feasibility check:

    if(work->v != NULL){
        /* existing Rinv / RinvD / identity branches */
    } else {
        // No linear term: unconstrained optimum is x = 0
        for(i = 0; i < n; i++) work->x[i] = 0.0;
    }

• interfaces/daqp-eigen/tests/08_unconstrained_check.cpp — Two new test cases:
  • min 0.5||x||² s.t. -1 ≤ x ≤ 1: x = 0 is feasible → shortcut must return x* = 0
  • min 0.5||x||² s.t. 2 ≤ x ≤ 5: x = 0 violates bounds → shortcut must not fire; solver returns x* = [2, 2]

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