drbdadm: fix segfault if we don't know about peer devices

Trying to do below "exploded" for me:
```
drbdsetup show > tmp
drbdadm -c tmp -d up all
```

With an incomplete config file (like the one obtained from "drbdsetup show"),
we may not know about peer volumes.  Avoid to dereference NULL volumes.

Author: lge

user/v9/drbdadm_main.c

@@ -2027,6 +2027,12 @@ int adm_peer_device(const struct cfg_ctx *ctx)
 	const char *argv[MAX_ARGS];
 	int argc = 0;
 
+	if (!vol) {
+		log_err("No volume?!\n");
+		if (dry_run)
+			return 0;
+		exit(E_THINKO);
+	}
 	peer_device = find_peer_device(conn, vol->vnr);
 	if (!peer_device) {
 		log_err("Could not find peer_device object!\n");
@@ -2371,7 +2377,9 @@ static int adm_up(const struct cfg_ctx *ctx)
 
 			tmp2_ctx = tmp_ctx;
 			tmp2_ctx.vol = volume_by_vnr(&conn->peer->volumes, peer_device->vnr);
-			schedule_deferred_cmd(&peer_device_options_cmd, &tmp2_ctx, dcmd, 0);
+			if (tmp2_ctx.vol)
+				schedule_deferred_cmd(&peer_device_options_cmd, &tmp2_ctx, dcmd, 0);
+			/* else: oops? incomplete config file? */
 		}
 	}
 

user/v9/drbdadm_postparse.c

@@ -482,7 +482,8 @@ bool peer_diskless(struct peer_device *peer_device)
 	struct d_volume *vol;
 
 	vol = volume_by_vnr(&peer_device->connection->peer->volumes, peer_device->vnr);
-	return vol->disk == NULL;
+	/* Do we even know? Maybe we have an incomplete config file. */
+	return vol ? vol->disk == NULL : false;
 }