should not use i in the inner loop, because the top-level loop also uses i

[yihui]

e.g. when data.evals.length == 1, i will be 1 by the end of this loop due to i++; then i will become 2 due to i++ from the outer loop, and the outer loop will exit early

here is a simplified minimal example, in which we expect i to be printed twice in the outer loop:

for (var i = 0; i < 2; i++) {
  console.log('outer loop i = ' + i);
  for (var i = 0; i < 1; i++) {
  }
  console.log('inner loop i = ' + i);
}
// outer loop i = 0
// inner loop i = 1
// i = 2 now

fixes rstudio/DT#25