When a process knote was attached to the process which is already exiting,
the knote is activated immediately. If the exit1() later activates
knotes, such knote is attempted to be activated second time. Detect
the condition by zeroed kn_ptr.p_proc pointer, and avoid excessive
activation.

Before r302235, such knotes were removed from the knlist immediately
upon activation.

Reported by: truckman
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)