loader: fix UB in wayland helper code.

The wayland helper will use uninitialized data, causing undefined
behaviour.

This was because the feedback data was allocated, but not fully
initialized.

After the fix, SDL's vulkantest runs cleanly through valgrind. Before,
it would not.

Signed-off-by: Bram Stolk <b.stolk@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39133>

src/loader/loader_wayland_helper.c

@@ -412,7 +412,8 @@ loader_wayland_presentation_feedback(struct loader_wayland_presentation *pres,
        !tracing)
       return;
 
-   fd = malloc(sizeof *fd);
+   fd = calloc(1, sizeof *fd);
+
    fd->presentation = pres;
    fd->tracing = tracing;
    if (tracing) {