Debug client crashes during the launch due to an assert in graphics engine

[kimden]

Crash doesn't happen when using RelWithDebInfo or Release. Crash happens to servers.
The kart that causes the crash is addon_bunnysky. Assert seems to be triggered if and only if the kart is installed (tested with all other karts present and absent).

Steps to reproduce

1. Install bunnysky addon
2. Compile STK with cmake .. -DCMAKE_BUILD_TYPE=Debug -DNO_SHADERC=OFF
3. Run the game

Configuration

STK release version: git e7dd96a

Additional information

stdout.log:

...
[warn   ] KartModel: brdm-2.spm has no striaght frame defined.
[warn   ] KartModel: breezyblue.spm has no striaght frame defined.
[warn   ] Material: Cannot determine texture full path: HY_shadow.png
[warn   ] KartModel: breezyblue.spm has no striaght frame defined.
[error  ] [KartProperties]: Kart 'Bublle racer' has an invalid engine 'medium'.
[warn   ] KartModel: buff tune-up.spm has no striaght frame defined.
[warn   ] KartProperties: Can't find kart type '' for kart 'Buggie', defaulting to 'light'.
[warn   ] Material: Cannot determine texture full path: semishiny_map3.png
[warn   ] Material: Cannot determine texture full path: matte_map2
supertuxkart: /folder/lib/graphics_engine/include/ge_animation.hpp:93: void GE::Armature::read(irr::io::IReadFile*): Assertion `m_joint_used > 0' failed.
Aborted (core dumped)

Backtrace:

(gdb) bt
#0  __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#3  0x00007452c6c428e6 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007452c6c268b7 in __GI_abort () at ./stdlib/abort.c:79
#5  0x00007452c6c267db in __assert_fail_base (fmt=0x7452c6dc5168 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", 
    assertion=assertion@entry=0x5eaa14e8a27e "m_joint_used > 0", 
    file=file@entry=0x5eaa14e8a230 "/folder/lib/graphics_engine/include/ge_animation.hpp", line=line@entry=93, 
    function=function@entry=0x5eaa14e8a200 "void GE::Armature::read(irr::io::IReadFile*)") at ./assert/assert.c:92
#6  0x00007452c6c39206 in __assert_fail (assertion=0x5eaa14e8a27e "m_joint_used > 0", 
    file=0x5eaa14e8a230 "/folder/lib/graphics_engine/include/ge_animation.hpp", line=93, 
    function=0x5eaa14e8a200 "void GE::Armature::read(irr::io::IReadFile*)") at ./assert/assert.c:101
#7  0x00005eaa143ea5b4 in GE::Armature::read (this=0x5eaa219eadd0, spm=0x5eaa21d6d720)
    at /folder/lib/graphics_engine/include/ge_animation.hpp:93
#8  0x00005eaa143e9174 in SPMeshLoader::createAnimationData (this=0x5eaa17bca540, spm=0x5eaa21d6d720)
    at /folder/src/graphics/sp_mesh_loader.cpp:708
#9  0x00005eaa143e716f in SPMeshLoader::createMesh (this=0x5eaa17bca540, f=0x5eaa21d6d720)
    at /folder/src/graphics/sp_mesh_loader.cpp:275
#10 0x00005eaa14b0841e in irr::scene::CSceneManager::getMesh (this=0x5eaa17959640, filename=...)
    at /folder/lib/irrlicht/source/Irrlicht/CSceneManager.cpp:213
#11 0x00005eaa142f1b89 in IrrDriver::getAnimatedMesh (this=0x5eaa1797f510, 
    filename="/userfolder/.local/share/supertuxkart/addons/karts/bunnysky/bunnysky.spm")
    at /folder/src/graphics/irr_driver.cpp:1269
#12 0x00005eaa145bfd04 in KartModel::loadModels (this=0x5eaa21a05300, kart_properties=...)
    at /folder/src/karts/kart_model.cpp:595
#13 0x00005eaa145d13cf in KartProperties::load (this=0x5eaa2190e500, 
    filename="/userfolder/.local/share/supertuxkart/addons/karts/bunnysky/kart.xml", node="kart")
    at /folder/src/karts/kart_properties.cpp:343
#14 0x00005eaa145cfe0d in KartProperties::KartProperties (this=0x5eaa2190e500, 
    filename="/userfolder/.local/share/supertuxkart/addons/karts/bunnysky/kart.xml")
    at /folder/src/karts/kart_properties.cpp:106
#15 0x00005eaa145dd17c in KartPropertiesManager::loadKart (this=0x5eaa19bf1570, 
    dir="/userfolder/.local/share/supertuxkart/addons/karts/bunnysky")
    at /folder/src/karts/kart_properties_manager.cpp:261
#16 0x00005eaa145dc29b in KartPropertiesManager::loadAllKarts (this=0x5eaa19bf1570, loading_icon=true)
    at /folder/src/karts/kart_properties_manager.cpp:178
#17 0x00005eaa1460820b in main (argc=1, argv=0x7ffea17e0838) at /folder/src/main.cpp:2288