gio-tool: Fix a minor memory leak when using gio-set with bytestrings

Tested using:
```sh
touch ~/foo
gio set ~/foo -t bytestring user::test "\x00\x00"
```
(it doesn’t matter that this fails; the bytestring is still decoded)

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Coverity CID: #1474407