qrtplib: clear buffer fed to RTPSession::CreateCNAME

The RTPSession::CreateCNAME function checks to see if the buffer that
it is provided already has any data in it, and appends to it if so. The
RTPSession::InternalCreate function calls this function with an uninitialized
buffer, which results in indeterminate behavior. To ensure that the CNAME
is properly created, we clear the buffer before use.

==30323== Conditional jump or move depends on uninitialised value(s)
==30323==    at 0x4C30109: __strlen_sse2 (vg_replace_strmem.c:460)
==30323==    by 0x85647A4: qrtplib::RTPSession::CreateCNAME(unsigned char*, unsigned long*, bool) (rtpsession.cpp:1150)
==30323==    by 0x8564B35: qrtplib::RTPSession::InternalCreate(qrtplib::RTPSessionParams const&) (rtpsession.cpp:218)
==30323==    by 0x5499159: RTPSink::RTPSink(QUdpSocket*, int, bool) (rtpsink.cpp:48)
==30323==    by 0x5420B6A: AudioNetSink::AudioNetSink(QObject*, int, bool) (audionetsink.cpp:42)
==30323==    by 0x541F465: AudioOutput::start(int, int) (audiooutput.cpp:114)
==30323==    by 0x5412763: AudioDeviceManager::startAudioOutput(int) (audiodevicemanager.cpp:361)
==30323==    by 0x5412B0C: AudioDeviceManager::addAudioSink(AudioFifo*, MessageQueue*, int) (audiodevicemanager.cpp:229)
==30323==    by 0x33F96DE7: BFMDemod::BFMDemod(DeviceSourceAPI*) (bfmdemod.cpp:56)
==30323==    by 0x33FB03F2: non-virtual thunk to BFMPlugin::createRxChannelBS(DeviceSourceAPI*) (bfmplugin.cpp:62)
==30323==    by 0x4F47F25: DeviceUISet::loadRxChannelSettings(Preset const*, PluginAPI*) (deviceuiset.cpp:199)
==30323==    by 0x4EA51EA: MainWindow::loadPresetSettings(Preset const*, int) (mainwindow.cpp:575)
==30323==    by 0x4EAC81B: MainWindow::MainWindow(qtwebapp::LoggerWithFile*, MainParser const&, QWidget*) (mainwindow.cpp:176)
==30323==    by 0x10A49B: runQtApplication(int, char**, qtwebapp::LoggerWithFile*) (main.cpp:120)
==30323==    by 0x109B38: main (main.cpp:131)

qrtplib/rtpsession.cpp

@@ -209,7 +209,7 @@ int RTPSession::InternalCreate(const RTPSessionParams &sessparams)
     // Init the RTCP packet builder
 
     double timestampunit = sessparams.GetOwnTimestampUnit();
-    uint8_t buf[1024];
+    uint8_t buf[1024] = {0};
     std::size_t buflen = 1024;
     std::string forcedcname = sessparams.GetCNAME();