RootAuxDynIO: Fix for schema evolution involving allocators.
If we need to evolve from a vector type with a custom allocator type to one without, then brInfo.tclass may not have an associated type_info. Handle this case without crashing.
If we need to evolve from a vector type with a custom allocator type to one without, then brInfo.tclass may not have an associated type_info. Handle this case without crashing.