Exhausted memory cause un-prepared long jump
  Issue proper message when PIVOT column is nullable
modified:
  storage/connect/mysql-test/connect/r/pivot.result
  storage/connect/mysql-test/connect/t/pivot.test
  storage/connect/plgdbsem.h
  storage/connect/tabpivot.cpp

- Prepare adding index_prev (not used yet)
modified:
  storage/connect/plgdbsem.h
  storage/connect/xindex.cpp
  storage/connect/xindex.h