Safe.sol is used to perform ERC20 transfers in the protocols contracts. It does not check for contract existence, meaning any call to an address with no bytecode (the address zero or an EOA) would not revert upon Safe.transfer.
If there is a market with an underlying token being an empty bytecode address, this means a user can initiate a position without actually transferring any token to Swivel - as all protocols except Aave do not check for the underlying token.
As only an admin can add a market, the risk of this issue affecting the protocols is low.
In Safe.transfer() and Safe.transferFrom, success is set to 1 if the call returns 1, which is the case if it made to an address with no bytecode.
Use extcodesize() in Safe.sol transfer functions to ensure the destination contract has bytecode.
