Throwing std::out_of_range when showing store entry

Command 'show' throws std::out_of_range when is called without passing arguments.

test.v:
```verilog
module test (a, b, c);
    input a, b;
    output c;
    assign c = a | b;
endmodule
```

```
cirkit> read_verilog --xmg test.v 
cirkit [xmg]> show
terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at
```
