When loading language pack, a check is made not to double entries. This
check was wrong, because it tried to use offsets as pointers to strings.
These offsets are later rewritten to actual pointers in remaining part
of LanguagePack::LanguagePack
This is mostly `NULL` -> `nullptr` in cpp files, but I only noticed
because `LanguagePack::GetObjectOverride` and
`LanguagePack::GetScenarioOverride` were returning false instead of real
pointer.
Also fixes some whitespaces, you can `w=1` to github's URL to make it
skip these.
