fix transfer invariant, credits to @holiman for finding the edge case

specs/core/0_beacon-chain.md

@@ -2416,6 +2416,9 @@ def process_transfer(state: BeaconState, transfer: Transfer) -> None:
         get_balance(state, transfer.sender) == transfer.amount + transfer.fee or
         get_balance(state, transfer.sender) >= transfer.amount + transfer.fee + MIN_DEPOSIT_AMOUNT
     )
+    # Since the sender may decide to send to themselves, and lose a fee to the proposer (could be someone else), 
+    #  the above invariant may be broken. Hence, disallow transfers to yourself.
+    assert transfer.sender != transfer.recipient
     # A transfer is valid in only one slot
     assert state.slot == transfer.slot
     # Only withdrawn or not-yet-deposited accounts can transfer