Segfault when moving a fader in the mixer

Hi,

Didn’t find any issue that could match the bug, sorry if it exists already.

I don’t know how to reproduce but that’s an error I haven’t seen before.

The song was playing and I wanted to move all the faders to zero, I moved a few and then this error popped when moving one:

```
Thread 1 "lmms" received signal SIGSEGV, Segmentation fault.
0x00007ffff7f6d5a7 in ?? () from /lib/x86_64-linux-gnu/libQt5Xml.so.5
(gdb) bt full
#0 0x00007ffff7f6d5a7 in () at /lib/x86_64-linux-gnu/libQt5Xml.so.5
#1 0x00007ffff7f6d6a2 in () at /lib/x86_64-linux-gnu/libQt5Xml.so.5
#2 0x00007ffff7f6ea99 in () at /lib/x86_64-linux-gnu/libQt5Xml.so.5
#3 0x00007ffff7f6d5f4 in () at /lib/x86_64-linux-gnu/libQt5Xml.so.5
#4 0x00007ffff7f6e959 in () at /lib/x86_64-linux-gnu/libQt5Xml.so.5
#5 0x00005555557ba008 in lmms::DataFile::~DataFile() (this=0x5555644e3ab0, __in_chrg=) at /usr/local/src/lmms/include/DataFile.h:67
#6 0x00005555558833ce in lmms::ProjectJournal::CheckPoint::~CheckPoint() (this=0x5555644e3aa8, __in_chrg=) at /usr/local/src/lmms/include/ProjectJournal.h:105
#7 0x0000555555883f67 in QVector::erase(lmms::ProjectJournal::CheckPoint*, lmms::ProjectJournal::CheckPoint*)
(this=0x55555697de80, abegin=0x5555644e3aa8, aend=0x5555644e3ae0) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qvector.h:928
moveBegin = 0x5555644e3ae0
moveEnd = 0x5555644e50c0
itemsToErase = 1
itemsUntouched = 0
#8 0x000055555588374a in QVector::remove(int, int) (this=0x55555697de80, i=0, n=1) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qvector.h:474
#9 0x0000555555882fdb in lmms::ProjectJournal::addJournalCheckPoint(lmms::JournallingObject*) (this=0x55555697de70, jo=0x55555e502f10)
at /usr/local/src/lmms/src/core/ProjectJournal.cpp:124
dataFile = { = { = {impl = 0x7fff7000b8b0}, }, _vptr.DataFile = 0x555555bec470 , m_fileName = {d = 0x5555639c9820}, m_content = { = {impl = 0x55557a2958c0}, }, m_head = { = {impl = 0x5555656c97e0}, }, m_type = lmms::DataFile::Type::JournalData, m_fileVersion = 27}
#10 0x00005555558549f7 in lmms::JournallingObject::addJournalCheckPoint() (this=0x55555e502f10) at /usr/local/src/lmms/src/core/JournallingObject.cpp:62
#11 0x0000555555a04ec2 in lmms::gui::Fader::mousePressEvent(QMouseEvent*) (this=0x555564c322a0, mouseEvent=0x7fffffffd360) at /usr/local/src/lmms/src/gui/widgets/Fader.cpp:166
thisModel = 0x55555e502ef0
#12 0x00007ffff79a4db8 in QWidget::event(QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#13 0x00007ffff7962fae in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#14 0x00007ffff796b552 in QApplication::notify(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x00007ffff66b16f8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#16 0x00007ffff796965e in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer&, bool, bool) ()
at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007ffff79bdbd8 in () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#18 0x00007ffff79c0f60 in () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#19 0x00007ffff7962fae in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#20 0x00007ffff66b16f8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#21 0x00007ffff6b3d3ed in QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) () at /lib/x86_64-linux-gnu/libQt5Gui.so.5
#22 0x00007ffff6b11cac in QWindowSystemInterface::sendWindowSystemEvents(QFlags) () at /lib/x86_64-linux-gnu/libQt5Gui.so.5
#23 0x00007ffff22fbeca in () at /lib/x86_64-linux-gnu/libQt5XcbQpa.so.5
#24 0x00007ffff55d47a9 in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#25 0x00007ffff55d4a38 in () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#26 0x00007ffff55d4acc in g_main_context_iteration () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#27 0x00007ffff6709836 in QEventDispatcherGlib::processEvents(QFlags) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#28 0x00007ffff66b017b in QEventLoop::exec(QFlags) () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#29 0x00007ffff66b82d6 in QCoreApplication::exec() () at /lib/x86_64-linux-gnu/libQt5Core.so.5
#30 0x00005555557b77fc in main(int, char**) (argc=1, argv=0x7fffffffe168) at /usr/local/src/lmms/src/core/main.cpp:1018
coreOnly = false
fullscreen = true
exitAfterImport = false
allowRoot = false
renderLoop = false
renderTracks = false
fileToLoad = {d = 0x7ffff6797b20 }
--Type for more, q to quit, c to continue without paging--
fileToImport = {d = 0x7ffff6797b20 }
renderOut = {d = 0x7ffff6797b20 }
profilerOutputFile = {d = 0x7ffff6797b20 }
configFile = {d = 0x7ffff6797b20 }
app = 0x555556416580
qs = {interpolation = lmms::AudioEngine::qualitySettings::Interpolation::SincFastest, oversampling = lmms::AudioEngine::qualitySettings::Oversampling::X2}
os = {m_sampleRate = 44100, m_bitRateSettings = {m_bitRate = 160, m_isVariableBitRate = false}, m_bitDepth = lmms::OutputSettings::BitDepth::Depth16Bit, m_stereoMode = lmms::OutputSettings::StereoMode::JointStereo, m_compressionLevel = 0.625}
eff = lmms::ProjectRenderer::ExportFileFormat::Wave
pos = {d = 0x5555566048e0}
sparam = {sched_priority = 50}
sa = {__sigaction_handler = {sa_handler = 0x1, sa_sigaction = 0x1}, sa_mask = {__val = {0, 0, 0, 0, 0, 18446744073709551608, 1, 140737488347496, 140737488347512, 93825007790944, 4160551332, 47, 93824997733248, 49, 18446744073709551448, 93825007790944}}, sa_flags = 4, sa_restorer = 0x7fffffffdc60}
destroyEngine = false
ret = 0
```

Start by reviewing src/core/ProjectJournal.cpp around addJournalCheckPoint(), then inspect src/gui/widgets/Fader.cpp at mousePressEvent() and include/DataFile.h around DataFile destruction. Try to reproduce the crash while moving mixer faders and use the supplied backtrace to trace journal checkpoint cleanup. Done means identifying and fixing the segfault and confirming fader movement no longer crashes.

Written by the indexing model from the issue text.