Fix downloads:// crash when using 3 slashes.

See https://lkft.validation.linaro.org/scheduler/job/3632758#L845

```python
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/lava_dispatcher/action.py", line 245, in run_actions
    new_connection = action.run(connection, action_max_end_time)
  File "/usr/lib/python3/dist-packages/lava_dispatcher/actions/deploy/download.py", line 770, in run
    filename += self.url.path
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'
```

Signed-off-by: Rémi Duraffort <remi.duraffort@linaro.org>
18 jobs for fix-downloads-crash in 16 minutes and 9 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Test
passed #137140
amd64
dispatcher-debian-10

00:02:28

passed #137142
amd64
dispatcher-debian-11

00:02:28

passed #137141
amd64
server-debian-10

00:02:58

passed #137143
amd64
server-debian-11

00:05:03

 
  Analyze
passed #137144
amd64-dind
bandit-sast

00:01:01

passed #137147
amd64
black

00:00:50

passed #137148
amd64
code_quality

00:00:24

passed #137153
amd64
codespell

00:00:22

passed #137149
amd64
coverage

00:06:55

61.11%
passed #137150
amd64
dockerfiles

00:00:19

passed #137145
amd64-dind
eslint-sast

00:00:26

passed #137152
amd64
pylint

00:02:37

passed #137151
amd64
schemas

00:00:35

passed #137146
amd64-dind
semgrep-sast

00:01:13

 
  Build
passed #137155
amd64
debian/10

00:01:24

passed #137154
amd64
doc

00:00:36

passed #137156
amd64-dind
docker-amd64-dispatcher

00:03:37

passed #137157
amd64-dind
docker-amd64-server

00:04:03