Fix dupsh NUL bytes and make it shorter

The `mov` can generate NUL bytes.

## Old

```
$ shellcraft i386.linux.dupsh 3
bb030000006a0359496a3f58cd8075f831c9f7e96a01fe0c24682f2f7368682f62696eb00b89e3cd80
```

## New

```
$ shellcraft i386.linux.dupsh 3
6a035b6a0359496a3f58cd8075f831c9f7e96a01fe0c24682f2f7368682f62696eb00b89e3cd80
```