Fix potential unsafe casting of transaction count

[fkrause98]

We're casting from u64 to u16 here:

pub fn add_l2_to_l1_message(
    vm_state: &mut VMState,
    opcode: &Opcode,
    state_storage: &mut StateStorage,
) -> Result<(), EraVmError> {
    let key = vm_state.get_register(opcode.src0_index).value;
    let value = vm_state.get_register(opcode.src1_index).value;
    let is_service = opcode.imm0 == 1;
    state_storage.record_l2_to_l1_log(L2ToL1Log {
        key,
        value,
        is_service,
        address: vm_state.current_context()?.contract_address,
        shard_id: 0,
        tx_number: vm_state.tx_number as u16,  // <-------- THIS ONE
    })?;
    Ok(())
}

But the vm's field is actually a u64, so we should check if this is safe or simply change it to avoid bugs/panics.