cpp: Fix update of partial key in verifyresult

* lang/cpp/src/verificationresult.cpp
(Signature::key(bool,bool)): Don't update the returned copy
but the actual key of the signature.

--
The whole point of the update is to change the partial key
from the signature (e.g. only fingerprint and one uid as we
would have from tofu) to a fully keylisted one.

lang/cpp/src/verificationresult.cpp

@@ -413,7 +413,8 @@ GpgME::Key GpgME::Signature::key(bool search, bool update) const
         }
     }
     if (update) {
-        ret.update();
+        d->keys[idx].update();
+        ret = d->keys[idx];
     }
     return ret;
 }