Clean up dlobj cache only when no user is present

Clean up dlobj cache only when no user is present

Cleaning up the dlobj cache seems crashing some cases when the library
is used from another plugin like openal-soft.  A simple workaround is
to do the cleanup only when really no user is left, i.e. after all
close calls.

Bugzilla: https://bugzilla.novell.com/show_bug.cgi?id=814250

Signed-off-by: Takashi Iwai <tiwai@suse.de>