When Sub Emitters are used, memory is continuously leaked until the app crashes.

Steps to reproduce:
1) Open the latest attached project (ParticleLeakRepro.zip).
2) Build for any platform (Mac, Windows, iOS).
3) Run and monitor the memory usage. (I used Instruments with Allocations tool on macOS)

The scene consists of several Particle Systems with Sub Emitters. Memory is leaked continuously when running this scene. On macOS, for example DynamicHeapAllocator<LowLevelAllocator>::Allocate(unsigned long, int) allocates 4 MB every ~30 seconds (starting at around 00:01:30). On iOS, 64 KB are allocated on every iteration of the Particle System playback, leading to a crash due to memory warnings.

- Pressing the 'Toggle Sub Emitters' UI button stops the leaking immediately.

Reproduced on:
5.5.0p2, 5.5.0f3

Did not reproduce:
5.6.0b1, 5.4.3p3

Devices tested:
MacBook Pro 2013 macOS sierra 10.12.1
iPhone 7 iOS 10.2.1
Should also occur on other platforms as well.