Fix race condition causing an NPE in tlschannel

JAVA-3714

Author: rozza

driver-core/src/main/com/mongodb/internal/connection/tlschannel/async/AsynchronousTlsChannelGroup.java

@@ -121,7 +121,9 @@ class RegisteredSocket {
         public void close() {
             doCancelRead(this, null);
             doCancelWrite(this, null);
-            key.cancel();
+            if (key != null) {
+                key.cancel();
+            }
             currentRegistrations.getAndDecrement();
             /*
              * Actual de-registration from the selector will happen asynchronously.