Crashes on empty lines

When given an input file with an empty line, the application crashes with an uncaught exception.

```
$ echo "1\n2\n3\n" > crash.txt
$ untwister -i crash.txt
terminate called after throwing an instance of 'std::out_of_range'
what(): basic_string::at: __n (which is 0) >= this->size() (which is 0)
[1] 1160017 IOT instruction (core dumped) untwister -i crash.txt
```

```
$ echo "1\n\n2\n3\n4" > crash2.txt
$ untwister -i crash2.txt
terminate called after throwing an instance of 'std::out_of_range'
what(): basic_string::at: __n (which is 0) >= this->size() (which is 0)
[1] 1160344 IOT instruction (core dumped) untwister -i crash2.txt
```