Fix proxy assertion (#394)

* fix hash assertion

* remove comment

src/openzeppelin/upgrades/Proxy.cairo

@@ -2,7 +2,6 @@
 # OpenZeppelin Contracts for Cairo v0.2.0 (upgrades/Proxy.cairo)
 
 %lang starknet
-#%builtins pedersen range_check bitwise
 
 from starkware.cairo.common.cairo_builtins import HashBuiltin
 from starkware.starknet.common.syscalls import (

src/openzeppelin/upgrades/library.cairo

@@ -120,8 +120,10 @@ namespace Proxy:
             range_check_ptr
         }(new_implementation: felt):
         with_attr error_message("Proxy: implementation hash cannot be zero"):
-            Proxy_implementation_hash.write(new_implementation)
+            assert_not_zero(new_implementation)
         end
+
+        Proxy_implementation_hash.write(new_implementation)
         Upgraded.emit(new_implementation)
         return ()
     end

tests/upgrades/test_upgrades.py

@@ -203,6 +203,26 @@ async def test_upgrade_from_non_admin(proxy_factory):
     )
 
 
+@pytest.mark.asyncio
+async def test__set_implementation_as_zero(proxy_factory):
+    admin, _, proxy, _, _ = proxy_factory
+
+    # initialize implementation
+    await signer.send_transaction(
+        admin, proxy.contract_address, 'initializer', [
+            admin.contract_address
+        ]
+    )
+
+    # upgrade should revert
+    await assert_revert(
+        signer.send_transaction(
+            admin, proxy.contract_address, 'upgrade', [0]
+        ),
+        reverted_with="Proxy: implementation hash cannot be zero"
+    )
+
+
 @pytest.mark.asyncio
 async def test_implementation_v2(after_upgrade):
     admin, _, proxy, _, v2_decl = after_upgrade