update: free empty path string (memleak)

When you pass an empty string to directory_update_init(), it was not
freed by update_task().

src/update.c

@@ -767,7 +767,6 @@ static void * update_task(void *_path)
 {
 	if (_path != NULL && !isRootDirectory(_path)) {
 		updatePath((char *)_path);
-		g_free(_path);
 	} else {
 		struct directory *directory = db_get_root();
 		struct stat st;
@@ -776,6 +775,8 @@ static void * update_task(void *_path)
 			updateDirectory(directory, &st);
 	}
 
+	g_free(_path);
+
 	if (modified || !db_exists())
 		db_save();