using data_frame constructor after growing vectors results in incorrect number of rows

[jimhester]

cpp11::cpp_function('
data_frame foo() {
  writable::integers x, y;
  for (int i = 0; i < 10; ++i) {
    x.push_back(i);
    y.push_back(i);
  }
  return writable::data_frame({
    "x"_nm = x,
    "y"_nm = y
  });
}')
foo()
#>     x  y
#> 1   0  0
#> 2   1  1
#> 3   2  2
#> 4   3  3
#> 5   4  4
#> 6   5  5
#> 7   6  6
#> 8   7  7
#> 9   8  8
#> 10  9  9
#> 11 NA NA
#> 12 NA NA
#> 13 NA NA
#> 14 NA NA
#> 15 NA NA
#> 16 NA NA

^{Created on 2020-08-13 by the reprex package (v0.3.0)}

cc @thomasp85