Crash on bake pose when skeleton contains a virtual bone

When a skeleton contains a virtual bone, performing Bake Pose can crash the editor.

1. Create a third person project
2. Open SK_Mannequin
3. Check its Skeleton and add a virtual bone to root
4. Back to its Mesh and in Asset Details, LOD0 > LOD Info, Add some bones to" bones to remove" and select some animation in Bake Pose
5. Click Reapply remove bones

Assertion failed: (Index >= 0) & (Index < ArrayNum) [Link Removed] [Line: 610] Array index out of bounds: -1 from an array of size 69

UE4Editor_Core!FDebug::AssertFailed() [d:\p4\main\engine\source\runtime\core\private\misc\assertionmacros.cpp:419]
UE4Editor_MeshBoneReduction!FReferenceSkeleton::GetParentIndexInternal() [d:\p4\main\engine\source\runtime\engine\public\referenceskeleton.h:162]
UE4Editor_MeshBoneReduction!FMeshBoneReduction::RetrieveBoneMatrices() [d:\p4\main\engine\source\developer\meshbonereduction\private\meshbonereduction.cpp:312]
UE4Editor_MeshBoneReduction!FMeshBoneReduction::ReduceBoneCounts() [d:\p4\main\engine\source\developer\meshbonereduction\private\meshbonereduction.cpp:403]
UE4Editor_Persona!FPersonaMeshDetails::RemoveBones() [d:\p4\main\engine\source\editor\persona\private\personameshdetails.cpp:1590]
UE4Editor_Persona!TMemberFunctionCaller<FPersonaMeshDetails,FReply (__cdecl FPersonaMeshDetails::*)(int) __ptr64>::operator()<int const & __ptr64>() [d:\p4\main\engine\source\runtime\core\public\delegates\delegateinstanceinterface.h:156]
UE4Editor_Persona!UE4Tuple_Private::TTupleImpl<TIntegerSequence<unsigned int,0>,int>::ApplyAfter<TMemberFunctionCaller<FPersonaMeshDetails,FReply (__cdecl FPersonaMeshDetails::*)(int) __ptr64> >() [d:\p4\main\engine\source\runtime\core\public\templates\tuple.h:429]
UE4Editor_Persona!TBaseSPMethodDelegateInstance<0,FPersonaMeshDetails,0,FReply __cdecl(void),int>::Execute() [d:\p4\main\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:269]
UE4Editor_Slate!TBaseDelegate<FReply>::Execute() [d:\p4\main\engine\source\runtime\core\public\delegates\delegatesignatureimpl.inl:537]
UE4Editor_Slate!SButton::OnMouseButtonUp() [d:\p4\main\engine\source\runtime\slate\private\widgets\input\sbutton.cpp:282]
UE4Editor_Slate!<lambda_1002768c627006711ef2f351a87ec0e7>::operator()() [d:\p4\main\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5435]
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_1002768c627006711ef2f351a87ec0e7> >() [d:\p4\main\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:266]
UE4Editor_Slate!FSlateApplication::RoutePointerUpEvent() [d:\p4\main\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5424]
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() [d:\p4\main\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5922]
UE4Editor_Slate!FSlateApplication::OnMouseUp() [d:\p4\main\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5902]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessDeferredMessage() [d:\p4\main\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:1726]
UE4Editor_ApplicationCore!FWindowsApplication::DeferMessage() [d:\p4\main\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:2172]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessMessage() [d:\p4\main\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:889]
UE4Editor_ApplicationCore!FWindowsApplication::AppWndProc() [d:\p4\main\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:726]
user32
user32
UE4Editor_ApplicationCore!FWindowsPlatformApplicationMisc::PumpMessages() [d:\p4\main\engine\source\runtime\applicationcore\private\windows\windowsplatformapplicationmisc.cpp:129]
UE4Editor!FEngineLoop::Tick() [d:\p4\main\engine\source\runtime\launch\private\launchengineloop.cpp:3244]
UE4Editor!GuardedMain() [d:\p4\main\engine\source\runtime\launch\private\launch.cpp:166]
UE4Editor!GuardedMainWrapper() [d:\p4\main\engine\source\runtime\launch\private\windows\launchwindows.cpp:134]
UE4Editor!WinMain() [d:\p4\main\engine\source\runtime\launch\private\windows\launchwindows.cpp:210]
UE4Editor!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:253]
kernel32
ntdll