Lv2 MDA Piano causes an invalid write

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### Bug Summary

Lv2 MDA Piano causes an invalid write when notes are edited while played.

#### Steps to reproduce

Use Lv2 MDA Piano. Edit notes (volume) while playing them (start and stop a few times). Possibly it's not even required to edit notes.

#### Expected behavior

No invalid write.

#### Actual behavior

Invalid write.

#### Affected LMMS versions

Any.

#### Logs

Click to expand
Using vgdb (http://log.fundamental-code.com/2012/01/30/vgdb-uniting-valgrind-and-gdb.html),
valgrind notices the first invalid write inside mdaPiano, where is written directly behind the
output buffer.

$ cat -n src/mdaPiano.cpp | grep -C 5 381

   376        }

   377        comb[cpos] = l + r;

   378        ++cpos &= cmax;

   379        x = cdep * comb[cpos];  //stereo simulator

   380

   381        *out0++ = l + x;

   382        *out1++ = r - x;

   383      }

   384

   385      if(!end)

   386      {

valgrind:

==811656== Thread 10 Mixer::fifoWrite:

==811656== Invalid write of size 4

==811656==    at 0x1389C637: mdaPiano::processReplacing(float**, float**, int) (mdaPiano.cpp:381)

==811656==    by 0x1389DDF2: lvz_run (wrapper.cpp:166)

==811656==    by 0x3D7100: lilv_instance_run (lilv.h:1948)

==811656==    by 0x3D848E: Lv2Proc::run(short) (Lv2Proc.cpp:333)

==811656==    by 0x3CE9CC: Lv2ControlBase::run(short) (Lv2ControlBase.cpp:148)

==811656==    by 0x20B7C230: Lv2Instrument::play(std::array*) (Lv2Instrument.cpp:165)

==811656==    by 0x35F2CA: InstrumentPlayHandle::play(std::array*) (InstrumentPlayHandle.h:65)

==811656==    by 0x38BFD2: PlayHandle::doProcessing() (PlayHandle.cpp:58)

==811656==    by 0x35F0B0: ThreadableJob::process() (ThreadableJob.h:74)

==811656==    by 0x37D53C: MixerWorkerThread::JobQueue::run() (MixerWorkerThread.cpp:84)

==811656==    by 0x37D7A8: MixerWorkerThread::startAndWaitForJobs() (MixerWorkerThread.cpp:157)

==811656==    by 0x376266: Mixer::renderNextBuffer() (Mixer.cpp:389)

==811656==  Address 0x18299790 is 0 bytes after a block of size 1,024 alloc'd

==811656==    at 0x483ADEF: operator new(unsigned long) (vg_replace_malloc.c:342)

==811656==    by 0x3B8B5D: __gnu_cxx::new_allocator::allocate(unsigned long, void const*) (new_allocator.h:115)

==811656==    by 0x3B8AB1: std::allocator_traits >::allocate(std::allocator&, unsigned long) (alloc_traits.h:460)

==811656==    by 0x3B89FF: std::_Vector_base >::_M_allocate(unsigned long) (stl_vector.h:346)

==811656==    by 0x3B8906: std::_Vector_base >::_M_create_storage(unsigned long) (stl_vector.h:361)

==811656==    by 0x3B8722: std::_Vector_base >::_Vector_base(unsigned long, std::allocator const&) (stl_vector.h:305)

==811656==    by 0x3B8592: std::vector >::vector(unsigned long, std::allocator const&) (stl_vector.h:511)

==811656==    by 0x3D558B: Lv2Ports::Audio::Audio(unsigned long, bool) (Lv2Ports.cpp:305)

==811656==    by 0x3D943D: Lv2Proc::createPort(unsigned long) (Lv2Proc.cpp:585)

==811656==    by 0x3D9D16: Lv2Proc::createPorts() (Lv2Proc.cpp:723)

==811656==    by 0x3D8829: Lv2Proc::initPlugin() (Lv2Proc.cpp:393)

==811656==    by 0x3D7A0D: Lv2Proc::Lv2Proc(LilvPluginImpl const*, Model*) (Lv2Proc.cpp:164)

gdb says:

(gdb) bt

#0  mdaPiano::processReplacing(float**, float**, int) (this=0x18635c20, inputs=0x0, outputs=0xba82810, sampleFrames=256) at ../src/mdaPiano.cpp:381

#1  0x000000001389ddf3 in lvz_run(LV2_Handle, uint32_t) (instance=0x12d5d1c0, sample_count=256) at ../lvz/wrapper.cpp:166

#2  0x00000000003d7101 in lilv_instance_run(LilvInstance*, uint32_t) (instance=0x17a56f90, sample_count=256) at /usr/include/lilv-0/lilv/lilv.h:1948

#3  0x00000000003d848f in Lv2Proc::run(short) (this=0x97dacf0, frames=256) at /home/johannes/cprogs/lmms/master/src/core/lv2/Lv2Proc.cpp:333

#4  0x00000000003ce9cd in Lv2ControlBase::run(short) (this=0x218300e8, frames=256) at /home/johannes/cprogs/lmms/master/src/core/lv2/Lv2ControlBase.cpp:148

#5  0x0000000020b7c231 in Lv2Instrument::play(std::array*) (this=0x21830080, buf=0x21850080) at /home/johannes/cprogs/lmms/master/plugins/Lv2Instrument/Lv2Instru

#6  0x000000000035f2cb in InstrumentPlayHandle::play(std::array*) (this=0x16b166c0, _working_buffer=0x21850080) at /home/johannes/cprogs/lmms/master/include/Inst

#7  0x000000000038bfd3 in PlayHandle::doProcessing() (this=0x16b166c0) at /home/johannes/cprogs/lmms/master/src/core/PlayHandle.cpp:58

#8  0x000000000035f0b1 in ThreadableJob::process() (this=0x16b166c0) at /home/johannes/cprogs/lmms/master/include/ThreadableJob.h:74

#9  0x000000000037d53d in MixerWorkerThread::JobQueue::run() (this=0x707180 ) at /home/johannes/cprogs/lmms/master/src/core/MixerWorkerThr

#10 0x000000000037d7a9 in MixerWorkerThread::startAndWaitForJobs() () at /home/johannes/cprogs/lmms/master/src/core/MixerWorkerThread.cpp:157

#11 0x0000000000376267 in Mixer::renderNextBuffer() (this=0xb8225d0) at /home/johannes/cprogs/lmms/master/src/core/Mixer.cpp:389

#12 0x0000000000378d4f in Mixer::fifoWriter::run() (this=0x97e0bc0) at /home/johannes/cprogs/lmms/master/src/core/Mixer.cpp:1263

#13 0x0000000005f2df0f in  () at /usr/lib/libQt5Core.so.5

#14 0x00000000048963e9 in start_thread () at /usr/lib/libpthread.so.0

#15 0x00000000066ac293 in clone () at /usr/lib/libc.so.6

(gdb) info args 

this = 0x18635c20

inputs = 0x0

outputs = 0xba82810

sampleFrames = 256

(gdb) info locals

V = 0x18636054

end = true

out0 = 0x18299794

out1 = 0x18299bd0

frame = 256

frames = 101

v = 13

x = -0.0248893611

l = -0.204520881

r = -0.186344534

i = -9666

ev = 0x100b1c6c

Start with src/mdaPiano.cpp around line 381 and reproduce the Lv2 MDA Piano playback/editing sequence under Valgrind. Trace the output buffer allocation in Lv2Ports.cpp and the call path through lvz/wrapper.cpp and src/core/lv2/Lv2Proc.cpp; done means the reproduction no longer reports an invalid write.

Written by the indexing model from the issue text.