DATV: more robust handling of TV screen renderer at destruction time

plugins/channelrx/demoddatv/datvdemodgui.cpp

@@ -266,6 +266,7 @@ DATVDemodGUI::DATVDemodGUI(PluginAPI* objPluginAPI, DeviceUISet *deviceUISet, Ba
 
 DATVDemodGUI::~DATVDemodGUI()
 {
+    m_objDATVDemod->stop();
     m_objDATVDemod->SetTVScreen(nullptr);
     m_objDATVDemod->setMERLabel(nullptr);
     m_objDATVDemod->setCNRLabel(nullptr);

plugins/channelrx/demoddatv/datvideorender.h

@@ -208,7 +208,7 @@ class DATVideoRenderThread : public QThread
             return;
         }
 
-        while (m_renderingVideo == true)
+        while ((m_renderingVideo == true) && (m_renderer))
         {
             if (!m_renderer->RenderStream()) {
                 break;