do not decrements countdown when that is 0

Change-Id: If3eee28eae24bb9fd45e65b426cd7bd7c3b26c2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1534210
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Simon Que <sque@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>

ninjalog_uploader_wrapper.py

@@ -21,7 +21,7 @@ def LoadConfig():
         with open(CONFIG, 'rb') as f:
             config = json.load(f)
             if config['version'] == VERSION:
-                config['countdown'] -= 1
+                config['countdown'] = max(0, config['countdown'] - 1)
                 return config
 
     return {