Segfault on MIDI mapping preset apply/load

[Sean M. Pappalardo]

Happens on trunk r2384. A segmentation fault occurs in a specific case when MidiScriptEngine::getScriptFunctions() tries to lock its mutex during MidiMapping::loadPreset(). (See attached backtrace.)

Steps to establish test conditions:
1) Start Mixxx with a MIDI controller attached
2) Activate it in the preferences (no need to load a mapping)
3) Close Mixxx so that state is saved.

Steps to reproduce:
1) Start Mixxx with a MIDI controller attached that had been activated on the last run.
2) Open preferences
3) Select that controller and de-activate it.
4) Click OK.
5) Re-open preferences
6) Select that controller
7) Activate it, and immediately load a mapping for it (whether it uses scripting or not)
8) Observe the crash

[RJ Skerry-Ryan (rryan)]

MidiMapping::m_pScriptEngine is NULL and there is no check for it on line 591 of midimapping.cpp. The backtrace clearly shows the MidiScriptEngine this pointer to be 0x0.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/5389