We have the check function, have it also detect blocks that are all zero
instead of leaving that to callers.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f9ad302e6b7dbcd1dff98d94ec5500ce27bebe10.1649685475.git.jani.nikula@intel.com