mtd: mtdconcat: prevent a read from eraseregions[-1]

If the erase region was found in the first iteration we read from
eraseregions[-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>

drivers/mtd/mtdconcat.c

@@ -427,7 +427,7 @@ static int concat_erase(struct mtd_info *mtd, struct erase_info *instr)
 		 * to-be-erased area begins. Verify that the starting
 		 * offset is aligned to this region's erase size:
 		 */
-		if (instr->addr & (erase_regions[i].erasesize - 1))
+		if (i < 0 || instr->addr & (erase_regions[i].erasesize - 1))
 			return -EINVAL;
 
 		/*
@@ -440,8 +440,8 @@ static int concat_erase(struct mtd_info *mtd, struct erase_info *instr)
 		/*
 		 * check if the ending offset is aligned to this region's erase size
 		 */
-		if ((instr->addr + instr->len) & (erase_regions[i].erasesize -
-						  1))
+		if (i < 0 || ((instr->addr + instr->len) &
+					(erase_regions[i].erasesize - 1)))
 			return -EINVAL;
 	}