compositor-drm: Prevent a crash in the pixman renderer

When pixman is used and no connector could be found (or any other
error), drm_backend_create() tried to destroy a gbm_device that would
only be created in init_egl(), resulting in a segfault.

Signed-off-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor-drm.c

@@ -3191,6 +3191,7 @@ err_drm_source:
 err_udev_input:
 	udev_input_destroy(&b->input);
 err_sprite:
+	if (b->gbm)
 		gbm_device_destroy(b->gbm);
 	destroy_sprites(b);
 err_udev_dev: