[MTD] Don't oops when the RedBoot partition table is empty

This fixes a regression with the RedBoot parsing code introduced by
commit 0b47d654Signed-off-by: NMartin Michlmayr <tbm@cyrius.com>
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

drivers/mtd/redboot.c

@@ -122,7 +122,7 @@ static int parse_redboot_partitions(struct mtd_info *master,
 				}
 			}
 			break;
-		} else {
+		} else if (buf[i].size != -1) {
 			/* re-calculate of real numslots */
 			numslots = buf[i].size / sizeof(struct fis_image_desc);
 		}