Memory leak in OperatorEXXPW when ik > nks triggers early return #8022

Describe the bug

Summary

get_exx_potential and get_exx_stress_potential in source/source_pw/module_pwdft/op_pw_exx_pot.cpp allocate a CPU buffer (pot_cpu = new Real[npw]) but can return early or throw before the matching delete[], leaking the allocation.

Affected locations

• Early return: if (ik > nks) { return; } at line 52 leaks pot_cpu every time it triggers.
• GPU exception path: If cudaHostRegister fails, the throw at lines 213 / 403 leaks pot_cpu and also leaves the pointer registered.

Impact

• Leak size: sizeof(Real) * npw bytes per call (typically ~100 KB–1 MB).
• Trigger frequency: Any EXX calculation where ik > nks (e.g., certain k-parallel or restart paths). If this happens once per SCF step per k-point, the leak is small but unbounded over long runs.

Suggested fix

Move the allocation after the early-return guard, or wrap pot_cpu in a std::unique_ptr<Real[]>:

### Expected behavior

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### To Reproduce

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### Environment

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### Additional Context

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