misc: call queue cleanup function
misc: call queue cleanup function

This function was introduced in !53 with 7fdbb0f9 but was not called.
I've updated the function to avoid possible complains if the pointer
is NULL.

Fixes warning:
  ../atspi/atspi-misc.c:785:1: warning: ‘cleanup_deferred_message’ defined but not used [-Wunused-function]
  785 | cleanup_deferred_message(void)
      | ^~~~~~~~~~~~~~~~~~~~~~~~

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core