gui.c Fix error handling of for_each_stat_item().

gui.c Fix error handling of for_each_stat_item().

In status_post_select(), if read_nonblock() succeeds (ret >= 0)
but for_each_stat_item() fails (ret2 < 0), para_gui aborts with

	para_gui: error.h:639: para_strerror: Assertion `num > 0' failed.

This happens for example if the user is not allowed to connect to
para_audiod.

The problem is that the (non-positive) value -ret is passed to
para_strerror() as the error code rather than -ret2. This commit
fixes the bug. It was introduced one year ago in commit f840be8d
(gui: Introduce status_post_select()).