sd-journal: add missing 'error' handling

When we reach an empty array, there are at least two possibilities:
- journal file is corrupted,
- invalid index is requested.

We cannot distinguish them here, let's simply return earlier.

src/libsystemd/sd-journal/journal-file.c

@@ -2788,6 +2788,9 @@ static int generic_array_get(
                         return r;
 
                 k = journal_file_entry_array_n_items(f, o);
+                if (k == 0)
+                        return 0;
+
                 if (i < k)
                         break;