Avoid NULL dereference when typing a dot in a file without tags

[b4n]

@techee NULL check was removed in 292383c, was that really expected?
Other solution would be early return in autocomplete_scope() when doc->tm_file is NULL, but that would mean that there wouldn't be scope completion in unsaved files.

We need a fix ASAP, as it happens whenever typing . in an unsaved file, or a file with no tags parser. E.g. you can reproduce by opening a new geany geany -vi and typing a., boom.

[techee]

@b4n Ah, I assumed there's always a TMSourceFile available for any GeanyDocument (I assumed there would be a NONE type TMSourceFile for an untitled document). I just wanted to get rid of the -1 which correspons to LANG_AUTO that we shouldn't use. Using TM_PARSER_NONE is perfectly fine and logical in this case so your patch LGTM.

[b4n]

No, we only have a TMSourceFile when we actually have a parser setup. We don't even have a TMSourceFile before saving the first time (for now).