setup: free old value before setting `work_tree`

Before assigning to `data->work_tree` in `read_worktree_config()`, free
any value we might already have picked up, so that we do not leak it.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -411,6 +411,7 @@ static int read_worktree_config(const char *var, const char *value, void *vdata)
 	} else if (strcmp(var, "core.worktree") == 0) {
 		if (!value)
 			return config_error_nonbool(var);
+		free(data->work_tree);
 		data->work_tree = xstrdup(value);
 	}
 	return 0;