UBI: bugfix in ubi_leb_change()

Do not call 'ubi_wl_put_peb()' if the LEB was unmapped.
Reported-by: Gabor Loki <loki@inf.u-szeged.hu>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/eba.c

@@ -891,12 +891,14 @@ retry:
 		goto write_error;
 	}
 
+	if (vol->eba_tbl[lnum] >= 0) {
 		err = ubi_wl_put_peb(ubi, vol->eba_tbl[lnum], 1);
 		if (err) {
 			ubi_free_vid_hdr(ubi, vid_hdr);
 			leb_write_unlock(ubi, vol_id, lnum);
 			return err;
 		}
+	}
 
 	vol->eba_tbl[lnum] = pnum;
 	leb_write_unlock(ubi, vol_id, lnum);