Don't read upgrade progress file if size hasn't changed

apparently that is possible sometimes?  or perhaps just an issue on python 3?

tailbone/views/upgrades.py

@@ -343,12 +343,13 @@ class UpgradeView(MasterView):
         offset = session.get('stdout.offset', 0)
         if os.path.exists(path):
             size = os.path.getsize(path) - offset
-            with open(path, 'rb') as f:
-                f.seek(offset)
-                chunk = f.read(size)
-                data['stdout'] = chunk.decode('utf8').replace('\n', '<br />')
-            session['stdout.offset'] = offset + size
-            session.save()
+            if size:
+                with open(path, 'rb') as f:
+                    f.seek(offset)
+                    chunk = f.read(size)
+                    data['stdout'] = chunk.decode('utf8').replace('\n', '<br />')
+                session['stdout.offset'] = offset + size
+                session.save()
 
         return data