Fix ZeroDivisionError in fetch_artifact.py (#4299)

Fixes a ZeroDivisionError in the download_artifact function of
fetch_artifact.py. The error occured when attempting to calculate the
download progress percentage of an zero file size artifact.

src/clusterfuzz/_internal/platforms/android/fetch_artifact.py

@@ -112,8 +112,9 @@ def download_artifact(client, bid, target, attempt_id, name, output_directory,
       status, done = downloader.next_chunk()
       if status:
         size_completed = int(status.resumable_progress)
-        percent_completed = (size_completed * 100.0) / size
-        logs.info('%.1f%% complete.' % percent_completed)
+        if size != 0:
+          percent_completed = (size_completed * 100.0) / size
+          logs.info('%.1f%% complete.' % percent_completed)
 
   return output_path