Divide by zero in beam update can cause editor to crash in Cascade

The licensee has traced the issue back to a divide by zero in  UParticleModuleTypeDataBeam2::Update. If the Target and Source are the same then the beams magnitude will be zero. When there is a Noise module the TravelRatio will be the TrueMagnitude/FullMagnitude, and FullMagnitude will be zero.

I tested replacing float FullMagnitude = BeamData->Direction.Size(); with float FullMagnitude = FMath::Max(BeamData->Direction.Size(), 0.001f); and the crash did not occur (Shelved CL 9001727).

1. Open the attached particle system in the editor.
2.  In Beam Data set the speed to the default value of 10.0

RESULT

The editor will crash.

Assertion failed: CheckVertexCount <= Source.VertexCount [Link Removed] [Line: 4857]

UE4Editor_Core!FWindowsErrorOutputDevice::Serialize() [e:\_release4.23\engine\source\runtime\core\private\windows\windowserroroutputdevice.cpp:79]
UE4Editor_Core!FOutputDevice::LogfImpl() [e:\_release4.23\engine\source\runtime\core\private\misc\outputdevice.cpp:71]
UE4Editor_Core!AssertFailedImplV() [e:\_release4.23\engine\source\runtime\core\private\misc\assertionmacros.cpp:101]
UE4Editor_Core!FDebug::CheckVerifyFailedImpl() [e:\_release4.23\engine\source\runtime\core\private\misc\assertionmacros.cpp:445]
UE4Editor_Engine!FDynamicBeam2EmitterData::FillData_Noise() [e:\_release4.23\engine\source\runtime\engine\private\particles\particlesystemrender.cpp:4857]
UE4Editor_Engine!FDynamicBeam2EmitterData::GetDynamicMeshElementsEmitter() [e:\_release4.23\engine\source\runtime\engine\private\particles\particlesystemrender.cpp:2787]
UE4Editor_Engine!FParticleSystemSceneProxy::GetDynamicMeshElements() [e:\_release4.23\engine\source\runtime\engine\private\particles\particlesystemrender.cpp:7008]
UE4Editor_Renderer!FSceneRenderer::GatherDynamicMeshElements() [e:\_release4.23\engine\source\runtime\renderer\private\scenevisibility.cpp:2807]
UE4Editor_Renderer!FSceneRenderer::ComputeViewVisibility() [e:\_release4.23\engine\source\runtime\renderer\private\scenevisibility.cpp:3869]
UE4Editor_Renderer!FDeferredShadingSceneRenderer::InitViews() [e:\_release4.23\engine\source\runtime\renderer\private\scenevisibility.cpp:4157]
UE4Editor_Renderer!FDeferredShadingSceneRenderer::Render() [e:\_release4.23\engine\source\runtime\renderer\private\deferredshadingrenderer.cpp:934]
UE4Editor_Renderer!RenderViewFamily_RenderThread() [e:\_release4.23\engine\source\runtime\renderer\private\scenerendering.cpp:3242]
UE4Editor_Renderer!TGraphTask<TEnqueueUniqueRenderCommandType<`FRendererModule::BeginRenderingViewFamily'::`27'::FDrawSceneCommandName,<lambda_98974a148448e2faeb6b39d7ee44228d> > >::ExecuteTask() [e:\_release4.23\engine\source\runtime\core\public\async\taskgraphinterfaces.h:847]
UE4Editor_Core!FNamedTaskThread::ProcessTasksNamedThread() [e:\_release4.23\engine\source\runtime\core\private\async\taskgraph.cpp:686]
UE4Editor_Core!FNamedTaskThread::ProcessTasksUntilQuit() [e:\_release4.23\engine\source\runtime\core\private\async\taskgraph.cpp:583]
UE4Editor_RenderCore!RenderingThreadMain() [e:\_release4.23\engine\source\runtime\rendercore\private\renderingthread.cpp:340]
UE4Editor_RenderCore!FRenderingThread::Run() [e:\_release4.23\engine\source\runtime\rendercore\private\renderingthread.cpp:471]
UE4Editor_Core!FRunnableThreadWin::Run() [e:_release4.23\engine\source\runtime\core\private\windows\windowsrunnablethread.c