The invariant check in RGData::getRow fails producing errors with idbassert

WITH ROLLUP patch added an invariant check that RGData always knows the number of columns and fixed size columns part size.
However the most recent develop build fails the check with the error logged:
```
Sep 29 10:13:33 ip-172-31-24-140 Calpont[407192]: 33.449902 |0|0|0| E 00 CAL0000: /mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/rowgroup/rowgroup.h@2200: assertion 'columnCount == row->getColumnCount() && rowSize == row->getSize()' failed %%10%%
```
The behavior were observed both with a real workload scenario shared by one of the customers(ask me for the details) and test workload `select count(distinct) from t group by char_11` over a table with 9 000 000 values.