Fix minor valgrind issues

- Fix valgrind invalid write of 1 byte.
- Fix a few plugin related memory leaks.

src/core/pluginloader.h

@@ -30,6 +30,7 @@ struct uhub_plugin
 {
 	void* handle;
 	plugin_unregister_f unregister;
+	char* filename;
 };
 
 struct uhub_plugins