[disco] Fix memory leak in ServiceDiscoveryManager

The lambda we schedule in 25ms captures a strong reference to the
XMPPConnection. However the lambda is part of the scheduled action,
which we save in the renewEntityCapsScheduledAction field. This causes
a memory leak, since the ServiceDiscoveryManager now holds a strong
reference to its XMPPConnection.

Fix this by obtaining the strong reference to the XMPPConnection, if
one still exists, within the lambda.

Fixes SMACK-926.

Reported-by: Damian Minkov <damencho@jitsi.org>

smack-extensions/src/main/java/org/jivesoftware/smackx/disco/ServiceDiscoveryManager.java

@@ -941,9 +941,12 @@ public final class ServiceDiscoveryManager extends Manager {
             }
         }
 
-        final XMPPConnection connection = connection();
-
         renewEntityCapsScheduledAction = scheduleBlocking(() -> {
+            final XMPPConnection connection = connection();
+            if (connection == null) {
+                return;
+            }
+
             renewEntityCapsPerformed.incrementAndGet();
 
             DiscoverInfoBuilder discoverInfoBuilder = DiscoverInfo.builder("synthetized-disco-info-response")