CLEANUP: http_fetch: Use local `unique_id` variable in `smp_fetch_uniqueid()`

CLEANUP: http_fetch: Use local `unique_id` variable in `smp_fetch_uniqueid()`

Instead of relying on the implementation detail that
`stream_generate_unique_id()` will store the unique ID in `strm->unique_id` we
should use the returned value, especially since that one is already checked in
the `isttest()`.

Reviewed-by: Volker Dusch <github@wallbash.com>