Fix NULL pointer dereference in dbuf_prefetch_indirect_done()

When ZFS is built with assertions, a prefetch is done on a redacted
blkptr and dpa->dpa_dnode is NULL, we will have a NULL pointer
dereference in dbuf_prefetch_indirect_done().

Both Coverity and Clang's Static Analyzer caught this.

Reported-by: Coverity (CID 1524671)
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14210