Segfault on quit in project manager (memdelete<GDNativeSingletonDiscover>)

[akien-mga]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Mageia 6 x86_64, Godot master 9b9344c

Issue description:
When closing the project manager, Godot segfaults:

Thread 1 "godot-git" received signal SIGSEGV, Segmentation fault.
0x00000000021f982a in Object::_predelete (this=0x0) at core/object.cpp:384
384             _predelete_ok = 1;

(gdb) bt
#0  0x00000000021f982a in Object::_predelete (this=0x0) at core/object.cpp:384
#1  0x0000000002204048 in predelete_handler (p_object=0x0) at core/object.cpp:1774
#2  0x0000000000d935b8 in memdelete<GDNativeSingletonDiscover> (p_class=0x0) at core/os/memory.h:107
#3  0x0000000000d922b9 in unregister_gdnative_types () at modules/gdnative/register_types.cpp:252
#4  0x0000000000d839a2 in unregister_module_types () at modules/register_module_types.gen.cpp:129
#5  0x0000000000d46193 in Main::cleanup () at main/main.cpp:1714
#6  0x0000000000d1a5f0 in main (argc=1, argv=0x7fffffffda88) at platform/x11/godot_x11.cpp:55

Seems related to GDNative, cc @karroffel.

Steps to reproduce:

• Start Godot
• Close it

[karroffel]

Will take a look