Fix occasional error display bugs

From time to time instead of outputting the erroneous line, interpreter outputs following:
```
!!! internal error !!! unable to display line !!!
```

E.g. with this code:
```lua
f = x -> print(x)
f("Hello")
```
I get:
```
Parser error:
At character 6 line 1
 1 | !!! internal error !!! unable to display line !!!
     ^~
Unparseable expression or statement
```