receive: fix buffer leak when receiving invalid packets

For fastd versions before v20, this was just a memory leak (which could
still be used for DoS, as it's remotely triggerable). With the new
buffer management of fastd v20, this will trigger an assertion failure
instead as soon as the buffer pool is empty.

src/receive.c

@@ -169,6 +169,11 @@ static inline void handle_socket_receive_known(
 
 	case PACKET_HANDSHAKE:
 		fastd_handshake_handle(sock, local_addr, remote_addr, peer, buffer);
+		break;
+
+	default:
+		fastd_buffer_free(buffer);
+		pr_debug("received packet with invalid type from %P[%I]", peer, remote_addr);
 	}
 }
 
@@ -195,6 +200,11 @@ static inline void handle_socket_receive_unknown(
 
 	case PACKET_HANDSHAKE:
 		fastd_handshake_handle(sock, local_addr, remote_addr, NULL, buffer);
+		break;
+
+	default:
+		fastd_buffer_free(buffer);
+		pr_debug("received packet with invalid type from unknown address %I", remote_addr);
 	}
 }