mtd: ensure index is positive

The index is signed, make sure it is not negative
when we read the array element.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 467a4f1..c356c0a 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -447,7 +447,7 @@
 		for (i=0; i< MAX_MTD_DEVICES; i++)
 			if (mtd_table[i] == mtd)
 				ret = mtd_table[i];
-	} else if (num < MAX_MTD_DEVICES) {
+	} else if (num >= 0 && num < MAX_MTD_DEVICES) {
 		ret = mtd_table[num];
 		if (mtd && mtd != ret)
 			ret = NULL;