wireless_timeline: make sure the hash table is always allocated.

At least on my Mac, if I start up Wireshark, start a capture
(non-monitor-mode) on the Wi-Fi adapter, add a comment to the SHB and
the first packet while it's capturing, stop the capture, and try to save
it, it warns that the wireless timeline hash table pointer is null.

Allocate it in the constructor.

ui/qt/widgets/wireless_timeline.cpp

@@ -328,7 +328,7 @@ WirelessTimeline::WirelessTimeline(QWidget *parent) : QWidget(parent)
     last = NULL;
     capfile = NULL;
 
-    radio_packet_list = NULL;
+    radio_packet_list = g_hash_table_new(g_direct_hash, g_direct_equal);
     connect(wsApp, SIGNAL(appInitialized()), this, SLOT(appInitialized()));
 }