glMemoryBarrier comes from ARB_shader_image_load_store, so on a desktop context
below GL 4.2 the pointer is null. bSupportsBBox is set from
bSupportsFragmentStoresAndAtomics, which such a driver can still have, so saving
a state reads the bounding box and calls a null pointer.
UsePersistentStagingBuffers() in OGLTexture.cpp already guards the same call with
bSupportsImageLoadStore.
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