xf86drm: fix null pointer deref in drmGetBufInfo

If info.count is large, drmMalloc() / alloca() may fail, and the
resulting null pointer is not null checked before dereference.

Issue: https://gitlab.freedesktop.org/mesa/drm/-/issues/62



Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Alistair Delva <adelva@google.com>