DATV Demod can crash SDRangel when closed

[srcejon]

In SDRangel 6.7.0, if you close the DATV Demod channel, SDRangel can sometimes crash.

2021-03-31 20:53:33.653 (D) DATVDemod::~DATVDemod
2021-03-31 20:53:33.653 (D) DSPDeviceSourceEngine::removeSink: DATVDemod
2021-03-31 20:53:33.653 (D) DSPDeviceSourceEngine::handleSynchronousMessages: DSPRemoveBasebandSampleSink
--Type for more, q to quit, c to continue without paging--

Thread 1 "sdrangel" received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) up
#1 0x00007fffb6b885bf in DATVDemodGUI::~DATVDemodGUI (this=, this=)
at /opt/build/srcejon/sdrangel/plugins/channelrx/demoddatv/datvdemodgui.cpp:271
271 m_objDATVDemod->stop();

Have seen it happen on both Windows and Linux.

[f4exb]

m_objDATVDemod->stop(); should not be called in GUI destructor because this is already invoked in ~DATVDemod