[autotest] offloader handle non-exist host properly

When a dut no longer exist in the inventory, simply
let get_host_queue_entries return empty list rather
than hitting KeyError. This will lead to the directory
being uploade right away. Assume when a dut is removed,
there should be no jobs running on it.

BUG=chromium:480734
TEST=Locally applied on drones. Offloader stop crashing.

Change-Id: Ie5d71df511cac30bc5aebfaff7ba2fae1f6d35f1
Reviewed-on: https://chromium-review.googlesource.com/266984
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Fang Deng <dengfang07@gmail.com>
Tested-by: Fang Deng <dengfang07@gmail.com>
diff --git a/server/frontend.py b/server/frontend.py
index 82cfcfe..dc2a31f 100644
--- a/server/frontend.py
+++ b/server/frontend.py
@@ -304,7 +304,7 @@
             host_hash[host.hostname] = host
         for status in job_statuses:
             if status.host:
-                status.host = host_hash[status.host.hostname]
+                status.host = host_hash.get(status.host.hostname)
         # filter job statuses that have either host or meta_host
         return [status for status in job_statuses if (status.host or
                                                       status.meta_host)]