Fixes an obvious issue of calling arc_buf_destroy() on an
unallocated arc_buf.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Closes#9453