Add assert for oldname

Code cannot proceed if oldname would be NULL.
Since lvmetad currently doesn't use logging mechanism of lvm to report
internal errors - stay with current code style of lvmetad which uses
plain asserts for cases like this.

daemons/lvmetad/lvmetad-core.c

@@ -595,6 +595,7 @@ static int remove_metadata(lvmetad_state *s, const char *vgid, int update_pvids)
 
 	if (!old)
 		return 0;
+	assert(oldname);
 
 	if (update_pvids)
 		update_pvid_to_vgid(s, old, "#orphan", 0);