Disallow creating variables with invalid WML tag names (fixes #3877)

Variables are saved as WML elements, and as such, variable names must also
be valid WML tag names.

src/variable_info_detail.hpp

@@ -41,6 +41,11 @@ class vi_policy_const
 	static const config& get_child_at(const config& cfg, const std::string& key, int index)
 	{
 		assert(index >= 0);
+
+		if(!config::valid_tag(key)) {
+			throw invalid_variablename_exception();
+		}
+
 		// cfg.child_or_empty does not support index parameter
 		if(const config& child = cfg.child(key, index)) {
 			return child;
@@ -63,6 +68,11 @@ class vi_policy_create
 	static config& get_child_at(config& cfg, const std::string& key, int index)
 	{
 		assert(index >= 0);
+
+		if (!config::valid_tag(key)) {
+			throw invalid_variablename_exception();
+		}
+
 		// the 'create_if_not_existent' logic.
 		while(static_cast<int>(cfg.child_count(key)) <= index) {
 			cfg.add_child(key);