onscreen/native: Avoid dereferencing gbm.next_fb when not in MODE_GBM
Such as with MODE_EGL_DEVICE.
Closes: #3436 (closed)
Fixes: 40950f99 ("drm-buffer-gbm: Do not call ensure_fb_id from...")
Such as with MODE_EGL_DEVICE.
Closes: #3436 (closed)
Fixes: 40950f99 ("drm-buffer-gbm: Do not call ensure_fb_id from...")