Not really clear why that would only show with multiple caches... But
point cache system is beyond brittle anyway.
This fix solves the issue at two different levels:
* General safety check in `rna_Cache_info_length` that we do get a valid
`pid`.
* Forbid usage of this `PointCache.info` RNA property in any
diffing/LibOverride processing (since it's by definition runtime, volatile
data).
d34c4089f1