Fix coverity defects: CID 147565-147567

coverity scan CID:147567, Type:dereference null return value
coverity scan CID:147566, Type:dereference null return value
coverity scan CID:147565, Type:dereference null return value

Reviewed by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: cao.xuewen <cao.xuewen@zte.com.cn>
Closes #5166