Error handling of n_max=NA could be improved

[jfsalzmann]

I recently had a hard time debugging that my code made a wrong vroom::vroom() call with an edge case bug on my side where I supplied n_max = NA.

Instead of some error/exception behaviour from vroom, there was an R error thrown from here, line 33

  if (n_max < 0 || is.infinite(n_max)) {
    n_max <- -1
  }

Error in if (condition) { : missing value where TRUE/FALSE needed