Crash When Playing After Setting Size of Retainer Box to Minus

Retainer box has minus size occurs crash when PIE, standalone and package. 

Probably, we should add any codes for adjust size to 0 or more to  URetainerBox::SynchronizeProperties().

1. Create UMG and add RetainerBox to the UMG
2. Add 'Create Widget' and 'Add to Viewport' to Level BP
3. PIE
4. Crash  

> UE4Editor-D3D11RHI-Win64-Debug.dll!FD3D11DynamicRHI::RHISetViewport(unsigned int MinX, unsigned int MinY, float MinZ, unsigned int MaxX, unsigned int MaxY, float MaxZ) Line 245 C++
UE4Editor-RHI-Win64-Debug.dll!FRHICommandList::SetViewport(unsigned int MinX, unsigned int MinY, float MinZ, unsigned int MaxX, unsigned int MaxY, float MaxZ) Line 2334 C++
UE4Editor-Engine-Win64-Debug.dll!FTextureRenderTarget2DResource::UpdateDeferredResource(FRHICommandListImmediate & RHICmdList, bool bClearRenderTarget) Line 520 C++
UE4Editor-Engine-Win64-Debug.dll!UTextureRenderTarget2D::ResizeTarget::__l8::<lambda>(FRHICommandListImmediate & RHICmdList) Line 146 C++
UE4Editor-Engine-Win64-Debug.dll!TGraphTask<TEnqueueUniqueRenderCommandType<`UTextureRenderTarget2D::ResizeTarget'::`8'::ResizeRenderTargetName,void <lambda>(FRHICommandListImmediate &) > >::ExecuteTask(TArray<FBaseGraphTask *,TSizedDefaultAllocator<32> > & NewTasks, ENamedThreads::Type CurrentThread) Line 849 C++
UE4Editor-Core-Win64-Debug.dll!FNamedTaskThread::ProcessTasksNamedThread(int QueueIndex, bool bAllowStall) Line 686 C++
UE4Editor-Core-Win64-Debug.dll!FNamedTaskThread::ProcessTasksUntilQuit(int QueueIndex) Line 583 C++
UE4Editor-Core-Win64-Debug.dll!FTaskGraphImplementation::ProcessThreadUntilRequestReturn(ENamedThreads::Type CurrentThread) Line 1415 C++
UE4Editor-RenderCore-Win64-Debug.dll!RenderingThreadMain(FEvent * TaskGraphBoundSyncEvent) Line 340 C++
UE4Editor-RenderCore-Win64-Debug.dll!FRenderingThread::Run() Line 490 C++
UE4Editor-Core-Win64-Debug.dll!FRunnableThreadWin::Run() Line 94 C++
UE4Editor-Core-Win64-Debug.dll!FRunnableThreadWin::GuardedRun() Line 45 C++
UE4Editor-Core-Win64-Debug.dll!FRunnableThreadWin::_ThreadProc(void * pThis) Line 80 C++
kernel32.dll!000000007729556d() Unknown
ntdll.dll!00000000773f385d() Unknown

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