mtd: mtdpart: prevent a read from regions[-1]

If the erase region was found in the first iteration we read from
regions[-1]

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>