mtd: mtdswap: fix possible null dereference

This patch fixes the possible null dereference of the variable "oinfo"
Thanks to Coverity for spotting this.

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>