sort: errors on overflowing -k argument but shouldn't

[jfinkels]

Steps to reproduce:

printf "2\n1\n" | sort -k 18446744073709551616

What happens now: uutils sort terminates with an error message:

sort: failed to parse key '18446744073709551616': failed to parse field index '18446744073709551616': number too large to fit in target type

What I expected to happen: GNU sort successfully sorts the input without error:

1
2

Notes: this is causing a failure in the GNU test case tests/sort/sort.pl. I think GNU just uses the maximum possible value of usize if the argument would overflow.

[cakebaker]

Fixed in #7188