When index-debug-info is enabled and the store path being copied is a
regular file (not a directory), std::get_if<nar_index::Directory>
returns nullptr since the NAR root is a File variant. The loop then
immediately dereferences buildIdDir->contents on the null pointer,
causing a segfault.
Add a null check at the top of the loop to break early when the NAR
root is not a directory.
Change-Id: I3a6e792b84cc12c837ecaddf4fee889e1bcb6397
(cherry picked from commit 6c7ccc2588)
