Fix panic on a set tag without a value

[chuenchen309]

A set tag without a = crashes the program instead of returning the error kong already wrote for it.

The guard contradicts its own next line:

parts := strings.SplitN(set, "=", 2)
if len(parts) == 0 {
    return fmt.Errorf("set should be in the form key=value but got %q", set)
}
t.Vars[parts[0]] = parts[1]

strings.SplitN never returns a zero-length slice, so len(parts) == 0 is dead code and parts[1] indexes a length-1 slice whenever the tag has no =.

Reproduction

var cli struct {
    Flag string `set:"novalue"`
}
_, err := kong.New(&cli)

panic: runtime error: index out of range [1] with length 1

For comparison, another malformed tag on a sibling path returns cleanly, and the well-formed case is unaffected:

Flag string `set:"novalue"`   ->  panic: index out of range [1] with length 1
Flag string `short:"ab"`      ->  error: <anonymous struct>.Flag: invalid short flag name "ab": invalid rune
Flag string `set:"k=v"`       ->  nil

The README documents the tag as set:"K=V", so a missing = is an ordinary typo, and it reaches this through kong.New / kong.Must / kong.Parse.

Fix

Compare against len(parts) != 2, so the message on the line above can actually fire:

<anonymous struct>.Flag: set should be in the form key=value but got "novalue"

Added TestTagSetWithoutValueErrors alongside TestInvalidRuneErrors, which is the same shape. go test ./... passes; go vet reports the same 78 pre-existing struct-tag warnings as on master.

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Disclosure: this was found, written and verified by an AI coding agent (Claude Code) running on this account — the commit carries a Co-Authored-By trailer for it. The outputs above came from running the code, not reading it, but the agent ran them, not a person. Happy to adjust anything.