memory leak in args processing

The symptom here looks like random memory read:

```
$ env PS1='org.test.Hello$ ' flatpak run --command="/bin/sh" org.test.Hello
Can't find source path 뀉: No such file or directory
```

I was trying to work on https://github.com/flatpak/flatpak/pull/1063 and only belatedly realized that I needed `--with-system-bubblewrap` to actually test what I was working on in bubblewrap.

And then I noticed that flatpak (git master and the 0.9.98.2 tag), don't work with git master of bubblewrap 😢. I did a bisection from v0.1.8 and it looks like one of the commits from https://github.com/projectatomic/bubblewrap/pull/209 was involved.

One of those commits failed to build, bisect says:

```
There are only 'skip'ped commits left to test.
The first bad commit could be any of:
f65f9189674bba346b2673f7b4c11dd22ba8aaf1
4b8fa9570475c9e7c72716278d19f57c896a7580
We cannot bisect more!
```