Errors: Fallback to fprintf if OS singleton doesn't exist

Otherwise we would crash if something prints an error before init or
after destruction of the `OS` singleton which handles printing/logging.

(cherry picked from commit c83084f)

core/error_macros.cpp

@@ -74,7 +74,13 @@ void _err_print_error(const char *p_function, const char *p_file, int p_line, co
 }
 
 void _err_print_error(const char *p_function, const char *p_file, int p_line, const char *p_error, const char *p_message, ErrorHandlerType p_type) {
-	OS::get_singleton()->print_error(p_function, p_file, p_line, p_error, p_message, (Logger::ErrorType)p_type);
+	if (OS::get_singleton()) {
+		OS::get_singleton()->print_error(p_function, p_file, p_line, p_error, p_message, (Logger::ErrorType)p_type);
+	} else {
+		// Fallback if errors happen before OS init or after it's destroyed.
+		const char *err_details = (p_message && *p_message) ? p_message : p_error;
+		fprintf(stderr, "ERROR: %s\n   at: %s (%s:%i)\n", err_details, p_function, p_file, p_line);
+	}
 
 	_global_lock();
 	ErrorHandlerList *l = error_handler_list;