nix {cat,ls}: Add back missing checks for file descriptors

I didn't catch this during the review of #14273.
This fixes that mistake.

Author: xokdvium

src/nix/cat.cc

@@ -76,6 +76,8 @@ struct CmdCatNar : StoreCommand, MixCat
     void run(ref<Store> store) override
     {
         AutoCloseFD fd = open(narPath.c_str(), O_RDONLY);
+        if (!fd)
+            throw SysError("opening NAR file '%s'", narPath);
         auto source = FdSource{fd.get()};
         auto narAccessor = makeNarAccessor(source);
         auto listing = listNar(narAccessor, CanonPath::root, true);

src/nix/ls.cc

@@ -146,6 +146,8 @@ struct CmdLsNar : Command, MixLs
     void run() override
     {
         AutoCloseFD fd = open(narPath.c_str(), O_RDONLY);
+        if (!fd)
+            throw SysError("opening NAR file '%s'", narPath);
         auto source = FdSource{fd.get()};
         auto narAccessor = makeNarAccessor(source);
         auto listing = listNar(narAccessor, CanonPath::root, true);