radeonsi: check set_debug_callback before use

The callback might not be installed yet if si_create_context fails
early.

Fixes: 59a3f38ff6 ("radeonsi: clear the debug callback on ctx destroy")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35733>

src/gallium/drivers/radeonsi/si_pipe.c

@@ -193,7 +193,8 @@ static void si_destroy_context(struct pipe_context *context)
 {
    struct si_context *sctx = (struct si_context *)context;
 
-   context->set_debug_callback(context, NULL);
+   if (context->set_debug_callback)
+      context->set_debug_callback(context, NULL);
 
    util_unreference_framebuffer_state(&sctx->framebuffer.state);
    util_framebuffer_init(context, NULL, sctx->framebuffer.fb_cbufs, &sctx->framebuffer.fb_zsbuf);