client API: avoid lost wakeups

The commit linked below added temporary unlocking to update_prop(),
which is indirectly called by mpv_wait_event(). If an unlock happens,
and no property change event is returned, we must re-poll the event
queue. Rechecking the state on unlocks is basically a standard
requirement for code using condition variables.

Untested beyond a simple test.

Fixes: 065c307e8e7dbf