virglrenderer: Fix the logic to check the callbacks

Initialization should also fail if no callbacks are given,
otherwise de-referencing the pointer will lead to a crash
later.

Fixes Coverity ID: 1527222 "Dereference after null check"

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/virgl/virglrenderer/-/merge_requests/975>
1 file changed