libcstore: Fix null deref in writeDebugInfo for non-directory NARs

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

lix/libstore/binary-cache-store.cc

@@ -221,6 +221,7 @@ try {
 
         auto * buildIdDir = std::get_if<nar_index::Directory>(&narIndex);
         for (auto subdir : { "lib", "debug", ".build-id" }) {
+            if (!buildIdDir) break;
             // get returns nullptr subdir does not exist, and std::get_if propagates it.
             buildIdDir = std::get_if<nar_index::Directory>(get(buildIdDir->contents, subdir));
         }