diskfilter: Treat an unrecognized RAID level as an error

disk/diskfilter.c: In function 'grub_diskfilter_make_raid':
disk/diskfilter.c:847:21: error: 'totsize' may be used uninitialized in this function [-Werror=uninitialized]

Perhaps this should be an assert() instead of return NULL?

=== modified file 'grub-core/disk/diskfilter.c'
Index: grub/grub-core/disk/diskfilter.c
===================================================================
--- grub.orig/grub-core/disk/diskfilter.c	2012-02-04 15:42:05.511916000 -0600
+++ grub/grub-core/disk/diskfilter.c	2012-02-04 15:50:17.059270000 -0600
@@ -837,6 +837,9 @@ grub_diskfilter_make_raid (grub_size_t u
     case 6:
       totsize = (nmemb - level / 3) * disk_size;
       break;
+
+    default:
+      return NULL;
     }
 
   array = grub_diskfilter_get_vg_by_uuid (uuidlen, uuid);