fix restoreSession closing the session it just re-activated

[skartikey]

During reconnect, the restoreSession path calls ActivateSession with the existing session. ActivateSession's internal callback always calls CloseSession before setting the new session. Since the old and new session are the same object, this sends CloseSessionRequest with DeleteSubscriptions: true, destroying all subscriptions on the server.

Clear the session reference before calling ActivateSession, matching the fix applied to recreateSession in #700. The session object is still held by the local variable and gets re-set inside the callback.

Fixes #854

[skartikey]

Adding context from a production report. A user running Telegraf 1.38.2 with inputs.opcua_listener against a Siemens OpenPCS7 V9.2 server hit this exact bug. The connection dropped after ~25 minutes of low data-change activity, and the auto-reconnect sequence played out exactly as described in #854:

restoreSession -> ActivateSession (success) -> CloseSession (!!!) -> "session restored"
-> ReadRequest -> StatusBadSessionIDInvalid
-> recreateSession (success) -> TransferSubscriptions -> ERRF/EOF
-> restoreSubscriptions -> broken pipe
-> "no subscriptions to resume"

After this, the client was permanently disconnected with no further reconnection attempts. The fix here matches what #700 did for recreateSession and is the minimal correct change.

One question: should closeSession(nil) also be guarded with a debug log (e.g., dlog.Printf("skipping close of nil session")) to make future troubleshooting easier, or is the current silent return-early sufficient?