Error if onion payloads exceed max length on packet construction.

Error if onion payloads exceed max length on packet construction.

Ensure that if we call construct_onion_packet and friends where payloads are
too large for the allotted packet length, we'll fail to construct. Previously,
senders would happily construct invalid packets by array-shifting the final
node's HMAC out of the packet when adding an intermediate onion layer, causing
the receiver to error with "final payload provided for us as an intermediate
node."