Fix warning reported by Clang static code analyzer:

  block.c:3167:5: warning: Value stored to 'ret' is never read
      ret = bdrv_fill_options(&options, filename, &flags, &local_err);
      ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 462f5bcf
Reported-by: Clang Static Analyzer
Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200422133152.16770-2-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>