egl: handle NULL loaderPrivate in dri_is_thread_safe

egl: handle NULL loaderPrivate in dri_is_thread_safe

This can happen when a context is created from
loader_dri3_blit_context_get and glthread is enabled.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7949
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7960
Cc: mesa-stable
Acked-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20479>
(cherry picked from commit b111e8aa4940e8b155f38b5ef238c295ded8a6b4)