Incorrect handling of "--" in GNU mode

Next entries are considered options when `--` is encountered in GNU mode. However, there is no check to ensure that they exist.
In the following test case:
``` kotlin
val parser = ArgParser("").apply {
argument(ArgType.String, fullName = "")
prefixStyle = ArgParser.OptionPrefixStyle.GNU
}

val args = arrayOf("--")
parser.parse(args)
```
Many results can be considered correct:
- String "--" is interpreted as a delimiter between options and arguments, parsing is successful with no options and no arguments
- String "--" is interpreted as an argument and parsing is successful
- String "--" is interpreted as a start of the option and the parser fails with a descriptive error because no options are registered

In practice though, it throws an `ArrayOutOfBoundsException`.
Bug is found by fuzzing team @ PLAN Lab.

Start with the GNU-mode parsing path in kotlinx-cli and reproduce the issue using the Kotlin snippet in the report with args set to arrayOf("--"). Trace how the delimiter or option-start case handles the missing next entry, then add or update a regression test so parsing no longer throws ArrayOutOfBoundsException and instead follows one of the valid outcomes described.

Written by the indexing model from the issue text.