gstreamer: Set data to null if it's consumed

When the probe has HANDLED the data, we're expecting the caller to no
longer access it. Let's explicitly protect against that by setting the
data to NULL on return.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1537>

gstreamer/src/pad.rs

@@ -1045,6 +1045,7 @@ unsafe fn update_probe_info(
                 || data_type == Some(Event::static_type()) =>
             {
                 // Buffer or Event consumed by probe
+                (*info).data = ptr::null_mut();
             }
             other => panic!("Bad data for {data_type:?} pad probe returning Handled: {other:?}"),
         }