client: cancel read in wl_display_read_events() when last_error is set

Calling wl_display_read_events() after an error should be equivalent
to wl_display_cancel_read(), so that display state is consistent.

Thanks to Pekka Paalanen <pekka.paalanen@collabora.co.uk>
for pointing that out.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>