clients/simple-dmabuf-egl: Properly check for error in gbm_bo_get_handle_for_plane

gbm_bo_get_handle_for_plane returns handle.s32 == -1 on error, at least
for the Mesa dri implementation.

Reported-by: Marius Vlad <marius.vlad@collabora.com>
Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>