client: broadcast the right pthread_cond variable

In previous commit we removed unused variables. One of them was
pthread_cond_t that was formerly used when reading from display, but
later was (erroneously) made unused. This patch fixes this error
and is a fix for the failing test introduced few patches ago (tests:
test if thread can block on error)

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>