pad: Fix race between gst_element_remove_pad and state change

pad: Fix race between gst_element_remove_pad and state change

When going from READY to NULL all element pads are deactivated. If
simultaneously the pad is being removed from the element with
gst_element_remove_pad() and the pad is unparented, there is a race
where the deactivation will assert (g_critical) if the parent is lost at
the wrong time.

The proposed fix will check parent only once and retain it to avoid the
race.

https://bugzilla.gnome.org/show_bug.cgi?id=761912