AK: Fix OOB access in DuplexMemoryStream::offset_of()

This fixes an OOB access when the last read/written chunk is empty (as we _just_
started on a new chunk).
Also adds a test case to TestMemoryStream.
Found via human fuzzing in the shell:
```sh
for $(cat /dev/urandom) {
    clear
    match $it {
        ?* as (x) {
            echo $x
            sleep 1
        }
    }
}
```
would assert at some point.

0 files changed, 0 insertions, 0 deletions