Deleting soundfont from Fluid Synthesizer while in processing event.type of CTRL_PROGRAM may cause segfault

Repro steps: Open the synth window and go to fluid synth tab. While loading a big score, delete all existing soundfonts.

Result: SegFault in FluidS::Preset::loadSamples thread:

1 FluidS::Preset::loadSamples sfont.cpp 151 0x73dde5
2 FluidS::Channel::setPreset chan.cpp 224 0x73d15e
3 FluidS::Fluid::program_change fluid.cpp 344 0x735fc2
4 FluidS::Fluid::play fluid.cpp 181 0x7355f8
5 Ms::MasterSynthesizer::play msynthesizer.cpp 92 0x72cb39
6 Ms::Seq::putEvent seq.cpp 1429 0x4dc14a
7 Ms::Seq::processMessages seq.cpp 579 0x4d8ed2
8 Ms::Seq::process seq.cpp 745 0x4d9860
9 Ms::paCallback pa.cpp 50 0x681c0b
10 NonAdaptingProcess 0x65344e3d
11 PaUtil_EndBufferProcessing 0x6534602f
12 ProcessingThreadProc *4 0x653520c3
13 KERNEL32!BaseThreadInitThunk 0x762b62c4
14 ntdll!RtlSubscribeWnfStateChangeNotification 0x77a00fd9
15 ntdll!RtlSubscribeWnfStateChangeNotification 0x77a00fa4
16 ??

because it is trying to access data from a FluidS::Zone which has been deleted by another thread:
1 ntdll!RtlCompareMemoryUlong 0x77a226d0
2 ntdll!WinSqmEventWrite 0x77a32e2a
3 ?? 0x2a1dc118
4 ntdll!RtlFreeHeap 0x779e8bd8
5 ntdll!RtlpNtSetValueKey 0x77a7fa72
6 ntdll!RtlFreeHeap 0x779e995d
7 ntdll!RtlFreeHeap 0x779e8bd8
8 msvcrt!free 0x745377c5
9 ?? 0x22d80000
10 FluidS::Zone::~Zone sfont.cpp 409 0x73eec5
11 FluidS::Instrument::~Instrument sfont.cpp 436 0x73f0b1
12 FluidS::SFont::~SFont sfont.cpp 69 0x73d7f8
13 FluidS::SFont::~SFont sfont.cpp 71 0x73d89d
14 FluidS::Fluid::sfunload fluid.cpp 730 0x73768f
15 FluidS::Fluid::removeSoundFont fluid.cpp 675 0x737480
16 FluidGui::soundFontDeleteClicked fluidgui.cpp 152 0x746b1f
17 FluidGui::qt_static_metacall moc_fluidgui.cpp 182 0x749b29
18 ZN11QMetaObject8activateEP7QObjectiiPPv 0x68c03046
19 ZN15QAbstractButton7toggledEb 0x226c2d1f
20 ?? 0x26d79080
...

These two threads need to be mutually-exclusive in this situation. I notice that Fluid::removeSoundFont() does protect itself with a mutex.lock():

bool Fluid::removeSoundFont(const QString& s)
      {
      mutex.lock();
      foreach(Voice* v, activeVoices)
            v->off();
      SFont* sf = get_sfont_by_name(s);
      sfunload(sf->id());
      mutex.unlock();
      return true;
      }

But for some reason someone commented out the mutex lock in Preset::loadSamples():

void Preset::loadSamples()
      {
//      sfont->synth->mutex.lock();
      if (_global_zone && _global_zone->instrument) {
            Instrument* i = _global_zone->instrument;
            if (i->global_zone && i->global_zone->sample)
                  i->global_zone->sample->load();
            foreach(Zone* iz, i->zones)
                  iz->sample->load();
            }

      foreach(Zone* z, zones) {
            Instrument* i = z->instrument;
            if (i->global_zone && i->global_zone->sample)
                  i->global_zone->sample->load();
            foreach(Zone* iz, i->zones)
                  iz->sample->load();
            }
//      sfont->synth->mutex.unlock();
      }

According to https://github.com/musescore/MuseScore/blame/master/fluid/sfont.cpp that commenting out was done prior to werner's initial git commit. Without knowing the prior git history, I cannot say why that mutex lock was removed.