fixed logic surrounding subscription cancel

1) When channel is destroyed, but subscription not installed into a server
2) When subscription is destroyed, but has not been installed in server

src/ca/cac.cpp

@@ -658,8 +658,7 @@ baseNMIU * cac::destroyIO (
     if ( pIO ) {
         class netSubscription * pSubscr = pIO->isSubscription ();
         if ( pSubscr ) {
-            chan.getPIIU(guard)->subscriptionCancelRequest ( 
-                guard, chan, *pSubscr );  
+            pSubscr->unsubscribeIfRequired ( guard, chan );  
         }
 
         // this uninstalls from the list and destroys the IO

src/ca/nciu.cpp

@@ -83,15 +83,16 @@ void nciu::destroy (
     epicsGuard < epicsMutex > & callbackControlGuard, 
     epicsGuard < epicsMutex > & mutualExclusionGuard )
 {    
+    while ( baseNMIU * pNetIO = this->eventq.first () ) {
+        assert ( this->cacCtx.destroyIO ( 
+            callbackControlGuard, mutualExclusionGuard, 
+            pNetIO->getId (), *this ) );
+    }
+
     // if the claim reply has not returned yet then we will issue
     // the clear channel request to the server when the claim reply
     // arrives and there is no matching nciu in the client
     if ( this->channelNode::isInstalledInServer ( mutualExclusionGuard ) ) {
-        while ( baseNMIU * pNetIO = this->eventq.first () ) {
-            assert ( this->cacCtx.destroyIO ( 
-                callbackControlGuard, mutualExclusionGuard, 
-                pNetIO->getId (), *this ) );
-        }
         this->getPIIU(mutualExclusionGuard)->clearChannelRequest ( 
             mutualExclusionGuard, this->sid, this->id );
     }

src/ca/netIO.h

@@ -89,6 +89,8 @@ public:
         epicsGuard < epicsMutex > & ) const;
     void subscribeIfRequired (
         epicsGuard < epicsMutex > & guard, nciu & chan );
+    void unsubscribeIfRequired ( 
+        epicsGuard < epicsMutex > & guard, nciu & chan );
     void subscriptionUpdateIfRequired (
         epicsGuard < epicsMutex > &, nciu & );
 protected:

src/ca/netSubscription.cpp

@@ -170,6 +170,16 @@ void netSubscription::subscribeIfRequired (
     }
 }
 
+void netSubscription::unsubscribeIfRequired ( 
+    epicsGuard < epicsMutex > & guard, nciu & chan )
+{
+    if ( this->subscribed ) {
+        chan.getPIIU(guard)->subscriptionCancelRequest ( 
+            guard, chan, *this );
+        this->subscribed = false;
+    }
+}
+
 void netSubscription::subscriptionUpdateIfRequired (
     epicsGuard < epicsMutex > & guard, nciu & chan )
 {