A use-after-free issue caused by GPUParticle

A licensee reported a crash which was caused by use after free of FXSystem.

The callstack catched by the user is [Link Removed], and could be debuggable with this project [Link Removed] .

The repro steps for debugging are:

1. Open engine solution and put breakpoint at FFXSystem::~FFXSystem(), void FPSCPool::Cleanup() and void FFXSystem::RemoveGPUSimulation(FParticleSimulationGPU Simulation)*
2. Click the Play button and wait until the particle disappeared
3. Click the Stop button then goes into the breakpoint.

You could see the FFXSystem::IsPendingKill() is invoked after it's freed.