Check if link override entity is nil

[tonsa1]

Explained here: #86

df_possible_overrides_from_entity(Arena *arena, DF_Entity *entity) function keeps creating new strings because the override is not valid.
Adding a check for override fixes the issue.

[ryanfleury]

This issue should be resolved now, with a few of the related paths (notably df_entity_from_path and the file path map view) fixed. I've also manually merged the nil-check in the df_possible_overrides_from_path path.