Indirectly caused by ada45519a7, the actual issue has been there for
years in the PointerRNA diffing code, which was way too basic and unsafe
in non-liboverride case.
Also fixed a copy/paste assert mistake in related code (same thing was
checked twice, instead of also checking another condition).
Finally, make the more general `RNA_struct_equals` call handle properly
cases where one or both of the given PointerRNA operands are Null ones.
20a65fd7f4