Qt: fix use of uninitialized variable

Fixes error reported by UBSan:

    ui/qt/packet_list.cpp:537:13: runtime error: load of value 190, which is not a valid value for type 'bool'

The method is apparently called earlier for other timers, resulting in
an error on startup. Initialize rows_inserted_ to avoid warnings when
the event is triggered later.

Change-Id: Iad919d79264ff1c3c17c9458c869d584234fcd5d
Reviewed-on: https://code.wireshark.org/review/7575
Reviewed-by: Peter Wu <peter@lekensteyn.nl>

ui/qt/packet_list.cpp

@@ -229,7 +229,8 @@ PacketList::PacketList(QWidget *parent) :
     decode_as_(NULL),
     ctx_column_(-1),
     capture_in_progress_(false),
-    tail_timer_id_(0)
+    tail_timer_id_(0),
+    rows_inserted_(false)
 {
     QMenu *submenu, *subsubmenu;
     QAction *action;