Only 'sign place' when lnum > 0

`erlc` produces errors with line nr 0, e.g. when you try to compile with `lager_transform` but `lager` is not yet compiled. If it does, you get this error:

```
Error detected while processing function erlang_compiler#AutoRun..erlang_compiler#errors#SetList..erlang_compiler#errors#AddError..erlang_compiler#errors#AddSign:
line    2:
E885: Not possible to change sign ErlangError
```