fix crash when glDebugMessageCallback is 0 despite feature being there

Change-Id: I1e6986e1e56f78f10f4677f471d2bdea2231f787
Reviewed-on: https://gerrit.libreoffice.org/9331
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>