Add sanity-checking for WML attribute/tag/variable names to prevent invalid save files

- It's now an error to set an invalid variable
- If an invalid key or tag is somehow set, it won't be written to the file (with an error message in the log)
- There's an API function config::valid_id() which can be used to catch issues earlier.

src/config.cpp

@@ -480,6 +480,19 @@ config &config::operator=(config &&cfg)
 }
 #endif
 
+bool config::valid_id(const std::string id)
+{
+	if (id.empty()) {
+		return false;
+	}
+	BOOST_FOREACH(char c, id) {
+		if (!isalnum(c) && c != '_') {
+			return false;
+		}
+	}
+	return true;
+}
+
 bool config::has_attribute(const std::string &key) const
 {
 	check_valid();

src/config.hpp

@@ -103,7 +103,9 @@ public:
 	explicit config(const std::string &child);
 
 	~config();
-
+	
+	// Verifies that the string can be used as an attribute or tag name
+	static bool valid_id(std::string);
 
 #ifdef HAVE_CXX11
 	explicit operator bool() const

src/serialization/parser.cpp

@@ -581,11 +581,19 @@ static void write_internal(config const &cfg, std::ostream &out, std::string& te
 		throw config::error("Too many recursion levels in config write");
 
 	BOOST_FOREACH(const config::attribute &i, cfg.attribute_range()) {
+		if (!config::valid_id(i.first)) {
+			ERR_CF << "Config contains invalid attribute name '" << i.first << "', skipping...\n";
+			continue;
+		}
 		write_key_val(out, i.first, i.second, tab, textdomain);
 	}
 
 	BOOST_FOREACH(const config::any_child &item, cfg.all_children_range())
 	{
+		if (!config::valid_id(item.key)) {
+			ERR_CF << "Config contains invalid tag name '" << item.key << "', skipping...\n";
+			continue;
+		}
 		write_open_child(out, item.key, tab);
 		write_internal(item.cfg, out, textdomain, tab + 1);
 		write_close_child(out, item.key, tab);

src/variable_info.cpp

@@ -170,10 +170,7 @@ namespace
 		char* endptr;
 		int res = strtol(index_str, &endptr, 10);
 
-		if (*endptr != ']') {
-			res = 0;//default
-		}
-		if(res > int(game_config::max_loop))
+		if (*endptr != ']' || res > int(game_config::max_loop))
 		{
 			throw invalid_variablename_exception();
 		}
@@ -186,7 +183,11 @@ namespace
 		: public variable_info_visitor<vit, void>
 	{
 	public:
-		get_variable_key_visitor(const std::string& key) : key_(key) {}
+		get_variable_key_visitor(const std::string& key) : key_(key) {
+			if (!config::valid_id(key_)) {
+				throw invalid_variablename_exception();
+			}
+		}
 		void from_named(typename get_variable_key_visitor::param_type state) const
 		{
 			if(key_ == "length")