Fix how we check file size when reading stdout for upgrade

i guess sometimes we were getting a negative number there

tailbone/views/upgrades.py

@@ -343,7 +343,7 @@ class UpgradeView(MasterView):
         offset = session.get('stdout.offset', 0)
         if os.path.exists(path):
             size = os.path.getsize(path) - offset
-            if size:
+            if size > 0:
                 with open(path, 'rb') as f:
                     f.seek(offset)
                     chunk = f.read(size)