Quick fix for OOG in deep calls

After an OOG exception in a contract calling another contract the ("bytesConstRef") code references a wrong value (this is tested in "ABAcalls0" in tests/StateTests/stSystemOperationsTest.json). This PR fixes the problem for now by changing the type to "bytes", but a deeper investigation of the problem is needed, since the same type of problem may occur again somewhere else (e.g. data in ExtVMFace).
fev.code.empty()