fix in stat_ecdf for Inf and NA values

```stat_ecdf``` fails when at least one value in ```data$x``` is ```NA``` or ```Inf```:
- for ```NA```, the ```min```/```max``` become ```NA```, leading to an error in ```seq```
- for ```Inf``` the min/max become infinite, and the finite end of the range is the only finite point in the result of the ```seq```-call. 

The fix removes ```NA``` and ```Inf``` for the purpose of computing the x-range. Note that all values are still included in the call to ```ecdf``` (which removes NAs, but handles ```Inf```s correctly).