Improve slider

I made a couple changes that:

1. Avoid slider's initial value error
- Clean up slider values (integers only)

The reason we were getting the error was because `max(qtlChr$pos)` was returning `-Inf` when 'All Chromosomes' was selected. I fixed this with `max(qtlChr$pos, 1)`, which allows max to be 1 if `qtlChr$pos` is undefined.