In SIGNAL-IO-THREAD-EVENTS, we may not always find a dentry for all

In SIGNAL-IO-THREAD-EVENTS, we may not always find a dentry for all
the descriptors and modes listed.  Don't assume that we shall.

This case arises particularly when there is no thread blocked in
BLOCK-ON-IO-DESCRIPTOR and someone invokes the PROCESS-WAIT primitive.
This should fix the random (%RECORD-REF #F 3) error that I have seen
when gunzipping MIT Scheme's Info reference manual and it isn't in the
operating system's disk buffer cache, which led to a losing race.