flush: Reset f->task_thread.error

f->task_thread.error can be set during flushing, not resetting this can
lead to c->task_thread.first being increased after having already submitted
a frame post flushing. That's fine if it happens on the very first frame,
but if that's the case on any subsequent frame it will incur a wrong frame
ordering.
Now that a non-first frame will be considered as such, its tasks won't be
able to execute (since they depend on a truly previous frame considered as
being after) and c->task_thread.cur will be increased past that frame, with
no way of it being reset, eventually leading to a hang.