Qt: add initializer (CID 1328485).

Change-Id: I1ce4ddc8bfb66522d4e472c258de2f928d5c776f
Reviewed-on: https://code.wireshark.org/review/17298
Petri-Dish: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Dario Lombardo <lomato@gmail.com>

ui/qt/lte_rlc_graph_dialog.cpp

@@ -65,6 +65,7 @@ LteRlcGraphDialog::LteRlcGraphDialog(QWidget &parent, CaptureFile &cf, bool chan
     reseg_graph_(NULL),
     acks_graph_(NULL),
     nacks_graph_(NULL),
+    tracer_(NULL),
     packet_num_(0)
 {
     ui->setupUi(this);