Blender Git Loki

Fix T87090: VSE scrubbing locks up blender

Speed effect caused, that some raw frames are re-used for multiple
final frames. When cached final frame is freed due to memory being
full, it tried to free also lower level cached frames that were used
during compositing. Some lower level cached frames were already freed
by different final frame and `BLI_ghash_remove()` failed.

Check if key still exists in hash or if linked keys were overwritten
before removing them.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D10909