432354e6d4
Check that `self` and `self->callback` are defined. `self` can be set to `NULL` in `remove_listener`, and `self->callback` can be set to `NULL` inside `gst_amc_surface_texture_jni_set_on_frame_available_callback`. This can cause a segfault since the Java object can outlive the C object, and call the callback after `remove_listener` is called. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2024>