libsystemd-network: add assert about packet length

We reject too-short packets in client_receive_message_raw(), so
the packets that dhcp_packet_verify_headers() gets are of sufficient size.
But let's add an assert to clarify this for the reader.

Closes systemd#23223.

src/libsystemd-network/dhcp-packet.c

@@ -129,6 +129,7 @@ int dhcp_packet_verify_headers(DHCPPacket *packet, size_t len, bool checksum, ui
         size_t hdrlen;
 
         assert(packet);
+        assert(len >= sizeof(DHCPPacket));
 
         /* IP */