BUG: special.chndtr infinite loop (#3102)

The comments in `cdfchn.f` say that the search ranges of `X`, `DF` and `PNONC` are limited, but in fact they are not. So the `cdfchn` will get stuck in an infinite loop if the input is infinite, which results in #3102.
Tests are not added because they will not return if they fail.