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

drivers/mtd/onenand/onenand_base.c

@@ -2453,7 +2453,7 @@ static int onenand_block_by_block_erase(struct mtd_info *mtd,
 		len -= block_size;
 		addr += block_size;
 
-		if (addr == region_end) {
+		if (region && addr == region_end) {
 			if (!len)
 				break;
 			region++;