flag: simplify arg logic in parseOne

Fixes #21763.

Change-Id: I59ee4f24c8064df64d9ede11aac02bc7ce4995b3
Reviewed-on: https://go-review.googlesource.com/61491Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/flag/flag.go

@@ -839,7 +839,7 @@ func (f *FlagSet) parseOne() (bool, error) {
 		return false, nil
 	}
 	s := f.args[0]
-	if len(s) == 0 || s[0] != '-' || len(s) == 1 {
+	if len(s) < 2 || s[0] != '-' {
 		return false, nil
 	}
 	numMinuses := 1