Rollback of NewCode recovers the original code

This addresses #4130 together with the corresponding change in the test.

libethereum/State.cpp

@@ -450,8 +450,8 @@ bytes const& State::code(Address const& _addr) const
 
 void State::setNewCode(Address const& _address, bytes&& _code)
 {
+	m_changeLog.emplace_back(_address, code(_address));
 	m_cache[_address].setNewCode(std::move(_code));
-	m_changeLog.emplace_back(Change::NewCode, _address);
 }
 
 h256 State::codeHash(Address const& _a) const
@@ -512,7 +512,10 @@ void State::rollback(size_t _savepoint)
 			m_cache.erase(change.address);
 			break;
 		case Change::NewCode:
-			account.resetCode();
+			if (change.oldCode.empty())
+				account.resetCode();
+			else
+				account.setNewCode(std::move(change.oldCode));
 			break;
 		case Change::Touch:
 			account.untouch();

libethereum/State.h

@@ -128,16 +128,24 @@ struct Change
 	Address address;  ///< Changed account address.
 	u256 value;       ///< Change value, e.g. balance, storage.
 	u256 key;         ///< Storage key. Last because used only in one case.
+	bytes oldCode;    ///< Code overwritten by CREATE, empty except in case of address collision.
 
 	/// Helper constructor to make change log update more readable.
 	Change(Kind _kind, Address const& _addr, u256 const& _value = 0):
 			kind(_kind), address(_addr), value(_value)
-	{}
+	{
+		assert(_kind != NewCode); // For this the special constructor needs to be used.
+	}
 
 	/// Helper constructor especially for storage change log.
 	Change(Address const& _addr, u256 const& _key, u256 const& _value):
 			kind(Storage), address(_addr), value(_value), key(_key)
 	{}
+
+	/// Helper constructor especially for new code change log.
+	Change(Address const& _addr, bytes const& _oldCode):
+			kind(NewCode), address(_addr), oldCode(_oldCode)
+	{}
 };
 
 using ChangeLog = std::vector<Change>;