parsing: ban empty enum definition.

libsolidity/parsing/Parser.cpp

@@ -406,6 +406,8 @@ ASTPointer<EnumDefinition> Parser::parseEnumDefinition()
 		if (m_scanner->currentToken() != Token::Identifier)
 			fatalParserError(string("Expected Identifier after ','"));
 	}
+	if (members.size() == 0)
+		parserError({"enum with no members is not allowed."});
 
 	nodeFactory.markEndPosition();
 	expectToken(Token::RBrace);