Merge pull request #7 from aorumbayev/aorumbayev/contract-vulnerability

Fixing opt-in exploit vulnerability in manager contract

backend/nft_market/contracts/assets/manager.py

@@ -25,7 +25,7 @@ def __init__(self):
     def on_register(self):
         return Seq(
             [
-                # Set default values for user
+                Assert(Global.group_size() == Int(1)),
                 self.bid_price.put(Int(0)),
                 Return(Int(1)),
             ]