[VecOps] Interaction between memory adoption and clear yields wrong results #6796
Description
Describe the bug
Calling clear followed by push_back(x) on a RVec that is adopting memory results in the wrong value being "pushed back":
~ root -l (cern-root)
root [0] ROOT::RVec<int> v{1,2,3}
(ROOT::RVec<int> &) { 1, 2, 3 }
root [1] ROOT::RVec<int> v2(v.data(), v.size())
(ROOT::RVec<int> &) { 1, 2, 3 }
root [2] v2[0] = 0
(int) 0
root [3] v
(ROOT::RVec<int> &) { 0, 2, 3 }
root [4] v2.clear()
root [5] v
(ROOT::RVec<int> &) { 0, 2, 3 }
root [6] v2.push_back(42)
root [7] v2
(ROOT::RVec<int> &) { 0 } // should be 42!!
root [8] v
(ROOT::RVec<int> &) { 0, 2, 3 }