Temporary fix for segmentation faults in FwdState::serverClosed.

Temporary fix for segmentation faults in FwdState::serverClosed.

r13388 (cache_peer standby=N) moved noteUses() call from Comm to FwdState, to
avoid exposing Comm to pconn pools. Unfortunately, the closing handler does
not get a valid FD value when the closing callback shares the Connection
object with the code that called conn->close(). It gets -1. The FD of the
FwdState connection itself is already -1 at that point, for similar reasons.
The code thinks it got a matching FD and calls noteUses() with an invalid FD.

This temporary workaround prevents noteUses() calls when FD is unknown.
Without those calls, pconn usage statistics will be wrong. A different
long-term solution is needed.