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 openzfs#14210

module/zfs/dbuf.c

@@ -3300,10 +3300,10 @@ dbuf_prefetch_indirect_done(zio_t *zio, const zbookmark_phys_t *zb,
 	blkptr_t *bp = ((blkptr_t *)abuf->b_data) +
 	    P2PHASE(nextblkid, 1ULL << dpa->dpa_epbs);
 
-	ASSERT(!BP_IS_REDACTED(bp) ||
+	ASSERT(!BP_IS_REDACTED(bp) || (dpa->dpa_dnode &&
 	    dsl_dataset_feature_is_active(
 	    dpa->dpa_dnode->dn_objset->os_dsl_dataset,
-	    SPA_FEATURE_REDACTED_DATASETS));
+	    SPA_FEATURE_REDACTED_DATASETS)));
 	if (BP_IS_HOLE(bp) || BP_IS_REDACTED(bp)) {
 		dbuf_prefetch_fini(dpa, B_TRUE);
 	} else if (dpa->dpa_curlevel == dpa->dpa_zb.zb_level) {