Pageant: call signop_unlink from signop_free.

A user reported that the following sequence of events leads to Pageant
crashing:

 - load an encrypted key into Pageant for decryption later
 - attempt to use the key, so that Pageant prompts for the passphrase
 - before entering the passphrase, abort the attempt to use the
   key (e.g. by closing the PuTTY that was trying to use it)
 - now enter the passphrase at the Pageant prompt, once the need for
   it has gone away.