avdeinterlace: Don't assume that get_current_caps() returns non-NULL caps after has_current_caps()
Remove calls to gst_pad_has_current_caps() which then go on to call gst_pad_get_current_caps() as the caps can go to NULL in between. Instead just use gst_pad_get_current_caps() and check for NULL. https://bugzilla.gnome.org/show_bug.cgi?id=759539