demo driver crashes when sr_session_stop is called during sampling.

Backtrace:

#0  0x00007ffff6115028 in g_io_channel_shutdown () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#1  0x00007ffff63ebea5 in hw_dev_acquisition_stop (sdi=0xa70050, cb_data=0xa70050) at demo.c:499
#2  0x00007ffff63db428 in sr_session_stop () at session.c:361
#3  0x000000000043cb9c in pv::SigSession::stop_capture (this=0x7fffffffdd58)
    at /home/joel/workspace/sigrok/sigrok/pulseview/pv/sigsession.cpp:87
#4  0x000000000043ad39 in pv::MainWindow::run_stop (this=0x7fffffffdd30)
    at /home/joel/workspace/sigrok/sigrok/pulseview/pv/mainwindow.cpp:201
#5  0x000000000044d2fe in pv::MainWindow::qt_static_metacall (_o=0x7fffffffdd30, 
    _c=QMetaObject::InvokeMetaMethod, _id=5, _a=0x7fffffffc680)
    at /home/joel/workspace/sigrok/sigrok/pulseview/pv/moc_mainwindow.cxx:65
#6  0x00007ffff69b1f5f in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) ()
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#7  0x000000000044d6d1 in pv::SamplingBar::run_stop (this=0xa33e30)
    at /home/joel/workspace/sigrok/sigrok/pulseview/pv/moc_samplingbar.cxx:103
#8  0x000000000044d5a3 in pv::SamplingBar::qt_static_metacall (_o=0xa33e30, 
    _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fffffffc8b0)
    at /home/joel/workspace/sigrok/sigrok/pulseview/pv/moc_samplingbar.cxx:53                             
#9  0x00007ffff69b1f5f in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) ()             
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4                                                          
#10 0x00007ffff752b422 in QAbstractButton::clicked(bool) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4  
#11 0x00007ffff727ed2e in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                              
#12 0x00007ffff727f560 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                              
#13 0x00007ffff727f7cc in QAbstractButton::mouseReleaseEvent(QMouseEvent*) ()                             
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                                                           
#14 0x00007ffff733999a in QToolButton::mouseReleaseEvent(QMouseEvent*) ()                                 
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                                                           
#15 0x00007ffff6f10850 in QWidget::event(QEvent*) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4         
#16 0x00007ffff6ec0e9c in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()                        
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                                                           
#17 0x00007ffff6ec5b6b in QApplication::notify(QObject*, QEvent*) ()                                      
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                                                           
#18 0x00007ffff699c56e in QCoreApplication::notifyInternal(QObject*, QEvent*) ()                          
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4                                                          
#19 0x00007ffff6ec1cd3 in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                      
#20 0x00007ffff6f3f554 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                              
#21 0x00007ffff6f3e44f in QApplication::x11ProcessEvent(_XEvent*) ()                                      
   from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                                                           
#22 0x00007ffff6f65fa2 in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                              
#23 0x00007ffff6120ab5 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0          
#24 0x00007ffff6120de8 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0                               
#25 0x00007ffff6120ea4 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0         
#26 0x00007ffff69cabf6 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()   
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4                                                          
#27 0x00007ffff6f65c1e in ?? () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4                              
#28 0x00007ffff699b2bf in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()             
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#29 0x00007ffff699b548 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#30 0x00007ffff69a0708 in QCoreApplication::exec() () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
#31 0x0000000000431d0b in main (argc=1, argv=0x7fffffffdfa8)
    at /home/joel/workspace/sigrok/sigrok/pulseview/main.cpp:115