client: read_events should return -1 after an error
When a thread is sleeping, waiting until another thread read
from the display, it always returns 0. Even when an error
occured. In documentation stands:
"return 0 on success or -1 on error. In case of error errno will
be set accordingly"
So this is a fix for this.
Along with the read_events, fix a test so that it now complies
with this behaviour (and we have this tested)
Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>