Reflection capture crash in FindOrAllocateCubemapIndex

See UDN thread

Not reproduced locally. From the licensee:

well the minimal repro would be to open large persistent level with many streaming levels in it, and show/hide them (each level contains many reflection probes) - after long enough playing with show/hide in Levels window (say 2 - 3 minutes) it will eventually crash - remember to have 'realtime' enabled in viewport. (our average level have 20 - 30 reflection probes)

Assertion failed: CaptureIndex >= 0 [Link Removed] [Line: 811]
UE4Editor_Core!FDebug::AssertFailed() [d:\unreal\sources\ue4_15\engine\source\runtime\core\private\misc\assertionmacros.cpp:349]
UE4Editor_Renderer!FindOrAllocateCubemapIndex() [d:\unreal\sources\ue4_15\engine\source\runtime\renderer\private\reflectionenvironmentcapture.cpp:812]
UE4Editor_Renderer!CopyToSceneArray() [d:\unreal\sources\ue4_15\engine\source\runtime\renderer\private\reflectionenvironmentcapture.cpp:1450]
UE4Editor_Renderer!TGraphTask<FScene::UpdateReflectionCaptureContents'::22'::EURCMacro_CopyCommand>::ExecuteTask() [d:\unreal\sources\ue4_15\engine\source\runtime\core\public\async\taskgraphinterfaces.h:883]
UE4Editor_Core!FNamedTaskThread::ProcessTasksNamedThread() [d:\unreal\sources\ue4_15\engine\source\runtime\core\private\async\taskgraph.cpp:954]
UE4Editor_Core!FNamedTaskThread::ProcessTasksUntilQuit() [d:\unreal\sources\ue4_15\engine\source\runtime\core\private\async\taskgraph.cpp:701]
UE4Editor_RenderCore!RenderingThreadMain() [d:\unreal\sources\ue4_15\engine\source\runtime\rendercore\private\renderingthread.cpp:325]
UE4Editor_RenderCore!FRenderingThread::Run() [d:\unreal\sources\ue4_15\engine\source\runtime\rendercore\private\renderingthread.cpp:459]
UE4Editor_Core!FRunnableThreadWin::Run() [d:\unreal\sources\ue4_15\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:76]