refactor(flatfile): drop dead cursor_offset, guard inverted range

src/database_flatfile.c

@@ -718,7 +718,7 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
     if (state->total_lines == 0)
         return DB_RESPONSE_EMPTY;
     // n_entries can be 0 even when total_lines > 0 if every line in the
-    // file failed _ff_maybe_index_line (e.g. unparseable timestamps).
+    // file failed _ff_maybe_index_line (e.g. unparsable timestamps).
     // Subsequent backup logic dereferences entries[0] unconditionally,
     // which would segfault on a NULL entries array.
     if (state->n_entries == 0)