Commit d0d7545ec1 ("Unload robot movies on exit") intended to shorten
the lifetime of the loaded data, but failed to save the unique_ptr, so
the lifetime was shortened more than intended. Save the unique_ptr so
that the movies remain loaded.
Also, add a [[nodiscard]] annotation so that the compiler can warn if
this mistake is repeated.
Reported-by: Q3BFG10K <https://github.com/dxx-rebirth/dxx-rebirth/issues/599>
Fixes: d0d7545ec1 ("Unload robot movies on exit")
c26fd86450