Prevent buffer overflow when assigning to fixed-dimension xtensor with mismatched rank

[Alex-PLACET]

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Description

Addressing #2792

Assigning a higher-dimensional expression to a fixed-rank [xtensor<T, N>] caused a buffer overflow (stack smashing) in release builds:

auto a = xt::xtensor<float, 1>::from_shape({2});
a = xt::expand_dims(a, 0);  // expand_dims returns 2D view, shape {1, 2}
// buffer overflow: resize truncates shape {1,2} -> {1}, then copies 2 elements into 1 element buffer