nix ls-{nar,store}: Return offset of files in the NAR if known

E.g.

  $ nix ls-store --json --recursive --store https://cache.nixos.org /nix/store/b0w2hafndl09h64fhb86kw6bmhbmnpm1-blender-2.79 \
    | jq .entries.bin.entries.blender.narOffset
  400

1 files changed, 4 insertions, 3 deletions

diff --git a/src/libstore/fs-accessor.hh b/src/libstore/fs-accessor.hh
index a67e0775b978..f703e1d15404 100644
--- a/src/libstore/fs-accessor.hh
+++ b/src/libstore/fs-accessor.hh
@@ -13,9 +13,10 @@ public:
 
     struct Stat
     {
-        Type type;
-        uint64_t fileSize; // regular files only
-        bool isExecutable; // regular files only
+        Type type = tMissing;
+        uint64_t fileSize = 0; // regular files only
+        bool isExecutable = false; // regular files only
+        uint64_t narOffset = 0; // regular files only
     };
 
     virtual Stat stat(const Path & path) = 0;