Crash when differentiating some std::vector code in the reverse mode

[gojakuch]

reproducer:

#include "clad/Differentiator/Differentiator.h"
#include "clad/Differentiator/STLBuiltins.h"
#include <vector>
#include <iostream>

double fn(double x, double y) {
  std::vector<double> a;
  a.push_back(0); // the problem is with pushing literal values for some reason
  a[1] = x*x;
  return a[1];
}

int main(int argc, char* argv[]) {
    double dx=0, dy=0;
    auto df = clad::gradient(fn, "x, y");
    std::cout << fn(3, 4) << '\n';
    df.execute(3, 4, &dx, &dy);
    std::cout << dx << ' ' << dy << '\n';
}