Add some sanity checks to Lint()

Since this is the second bug relating to NA lines, can we add a check to `Lint()` to assert that `line` is not `NA`?
This should expedite bug hunts in these cases. WDYT?

_Originally posted by @AshesITR in https://github.com/r-lib/lintr/pull/1446#discussion_r917639707_

Unfortunately a simple `stopifnot(!is.na(line))` in `Lint()` caused test failures, so this may require a bit more care.