using shift in getopts triggers an assert

I have encountered a few issues when using getopts while testing a potential bug in dash and discovered a segmentation fault condition in mrsh doing the following:

```sh
#!/bin/sh --

while getopts :a: arg; do
case $arg in
a)
set -- "$@" attr="$OPTARG"
shift 2
esac
done

printf '[%s]' "$@"
```

```
mrsh -x example.sh -a foo -a bar
+ getopts :a: arg
+ set -- -a foo -a bar attr=foo
+ shift 2
+ getopts :a: arg
mrsh: ../getopt.c:13: _mrsh_getopt: Assertion `argv[argc] == NULL' failed.
```

Reproduce the assertion with the provided shell script and command, then start in getopt.c at _mrsh_getopt and the argv[argc] assertion. Trace how getopts handles the positional-parameter changes after shift 2. Done means the script completes without the segmentation fault or assertion failure and produces the expected printf output.

Written by the indexing model from the issue text.