[master] Corrected invalid read complaint in D2ClientMgr

valgrind reported an invalid D2ClientMgr is destroyed while in send mode
Added call to stopSender to D2ClientMgr destructor.

src/lib/dhcpsrv/d2_client.cc

@@ -148,6 +148,9 @@ D2ClientMgr::D2ClientMgr() : d2_client_config_(new D2ClientConfig()),
 }
 
 D2ClientMgr::~D2ClientMgr(){
+    if (name_change_sender_) {
+        stopSender();
+    }
 }
 
 void