Negative array index causes panic

[maja42]

When accessing an array like this:

arr[-1]

... I expect jsonpath to return an error. Instead it panics (range error).
In evaluable.go (line 105), only the array length (upper limit) is checked, but not if the index is negative.

[generikvault]

I've added some testcases for this issue. It seems to bne already fixed.
Sry for the slow response.