Fix crash in LocationFunc when the input has incomplete location information

[fingolfin]

Also change it to error if the argument is not a function, and to
return fail (instead of an empty string) if the location can not
be determined.

Finally, make the underlying kernel function GET_LOCATION_BODY resilient
about function bodies that store no location information at all,
as produced by the code which turns syntax tree object back into
functions. This previously crashed.

Fixes #4507

Note: we could also try to improve the syntax tree code to store "complete" location information, but I am not sure what would be appropriate, so I opted to not tackle this in this PR. But if @zickgraf is interested in having that, PRs and discussions are welcome :-)

[zickgraf]

Thanks for the fix!

Note: we could also try to improve the syntax tree code to store "complete" location information, but I am not sure what would be appropriate, so I opted to not tackle this in this PR. But if @zickgraf is interested in having that, PRs and discussions are welcome :-)

Of course that would be nice to have, but I currently do not need it: I try to do all my plausibility checks right after calling SYNTAX_TREE, where I still have a reference to the original function. And once I have transformed the syntax tree, a reference to the original line would not be helpful in most cases. So basically I'm fine with any output, as long as there is no segfault :D