mtd: onenand: add missing check

Coverity has reported that inside the function "onenand_block_by_block_erase()"
in onenand_base.c, we should add a check to prevent the incrementing of
possible NULL value for "region"

Signed-off-by: Maxin B. John <john.maxin@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>