Only try to get current job when device is running in all_devices()

For some reason yet unknown, calling `.current_job()` on a Device
object when its state is not "running" can cause some server errors.
This is likely to be due to some discrepancy in the database.  The
schedule.all_devices() XMLRPC call causes an HTTP502 when this
happens.  Work around this problem by only looking for the current job
of devices that are in the running state.
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>

lava_scheduler_app/api/__init__.py

@@ -310,15 +310,20 @@ class SchedulerAPI(ExposedV2API):
             health=Device.HEALTH_RETIRED
         )
 
+        devices_jobs = ((
+            dev,
+            dev.current_job() if dev.state == Device.STATE_RUNNING else None
+        ) for dev in devices_list)
+
         return [
             [
                 dev.hostname,
                 dev.device_type.name,
                 build_device_status_display(dev.state, dev.health),
-                dev.current_job().pk if dev.current_job() else None,
+                job.pk if job else None,
                 True,
             ]
-            for dev in devices_list
+            for dev, job in devices_jobs
         ]
 
     def all_device_types(self):