audiod: Avoid segfault on exit.

audiod: Avoid segfault on exit.

When para_audiod terminates, it calls clean_exit() which closes
all slots unconditionally, regardless of whether they are in use.
This may lead to a segfault due to an invalid read:

	==25253== Invalid read of size 4
	==25253==    at 0x40EBEA: close_slot (audiod.c:443)
	==25253==    by 0x40FCE8: clean_exit (audiod.c:1163)
	==25253==    by 0x40FD7D: signal_post_select (audiod.c:1014)
	==25253==    by 0x41490A: schedule (sched.c:65)
	==25253==    by 0x406BC3: main (audiod.c:1436)
	==25253==  Address 0x5c19d8 is not stack'd, malloc'd or (recently) free'd

This bug was introduced 9 month ago in commit eec23a69 (audiod:
Fix memory leak on exit: close slots). Fix it by calling
close_unused_slots(). This also simplifies clean_exit() a bit.