Only imsm arrays can be rebuild by dmraid

Others cause a segfault.

Fix that so they print a more helpful error messages.

From: Leonardo Chiquitto <lchiquitto@suse.com>
Reviewed-by: NeilBrown <neilb@suse.de>
References: bnc#716904
---
 lib/metadata/reconfig.c |    5 +++++
 1 file changed, 5 insertions(+)

--- 1.0.0.rc16.orig/lib/metadata/reconfig.c
+++ 1.0.0.rc16/lib/metadata/reconfig.c
@@ -552,6 +552,11 @@ _rebuild_raidset(struct lib_context *lc,
 	int driveRebuild = 1;
 
 	rs = find_group(lc, sub_rs);
+	if (!rs) {
+		log_print(lc, "Rebuild: array \"%s\" cannot be rebuilt by dmraid.\n",
+			  set_name);
+		return 1;
+	}
 
 	/* raid 0 cannot be rebuild - exit */
 	if (T_RAID0(sub_rs) && (!SETS(sub_rs))) {