Fix segfault when update is NULL

http://bugs.gentoo.org/119245
http://www.mail-archive.com/linux-raid@vger.kernel.org/msg03639.html

--- Assemble.c
+++ Assemble.c
@@ -219,7 +219,7 @@
 		}
 		if (dfd >= 0) close(dfd);
 
-		if (ident->uuid_set && (!update && strcmp(update, "uuid")!= 0) &&
+		if (ident->uuid_set && (!update || strcmp(update, "uuid")!= 0) &&
 		    (!super || same_uuid(info.uuid, ident->uuid, tst->ss->swapuuid)==0)) {
 			if ((inargv && verbose >= 0) || verbose > 0)
 				fprintf(stderr, Name ": %s has wrong uuid.\n",