Carla segmentation fault

### Bug Summary
Immediate segmentation fault happens when I add Carla Rack or Carla Patchbay. It might be either Carla or LMMS issue, I'll move it to falkTX's repo if I should.

OS: Ubuntu [18.04.4](https://releases.ubuntu.com/18.04.4/) LTS (Bionic)
Hardware: Lenovo IdeaPad 330S-15ARR
QT version: `5.9.5`
GCC version: `7.5.0`
Last commit in log: `a05306`

**UPD**: if anyone can tell me how to differentiate if wrong data is being fed into Carla Patchbay / Rack, or a current version causes an error by itself, please do.

#### Steps to reproduce
* Open Lmms build in GDB (outside works the same): `gdb lmms`
* Type `run` in GDB to proceed
* Add Carla Rack or Carla Patchbay as an instrument
* See LMMS frozen and GDB showing a log below

#### Expected behavior
Being able to add Carla Patchbay / Carla Rack.

#### Actual behavior
Segmentation fault.

#### Affected LMMS versions
1.2.1.696, local build.
Sorry, I can't try to reproduce this bug with stable-1.2 branch, because it doesn't seem to support my current Qt version:

```
Found unsuitable Qt version "5.9.5" from /usr/bin/qmake, this code requires Qt 4.x
```

#### Logs

Click to expand

[New Thread 0x7fff0423e700 (LWP 8192)]


Thread 29 "InstrumentLoade" received signal SIGSEGV, Segmentation fault.

[Switching to Thread 0x7fff0423e700 (LWP 8192)]

0x00007fff07bdefd7 in CarlaInstrument::CarlaInstrument(InstrumentTrack*, Plugin::Descriptor const*, bool) ()

   from /home/grid/tools/lmms/plugins/libcarlabase.so

Reproduce the crash using the reported LMMS build under GDB, then inspect the CarlaInstrument::CarlaInstrument entry point in plugins/libcarlabase.so. Compare the failure when adding Carla Rack or Carla Patchbay and determine whether LMMS or Carla is responsible; done means the cause is established and adding either instrument no longer causes a segmentation fault.

Written by the indexing model from the issue text.