btrfs: update comment and drop assertion in extent item lookup in find_parent_nodes()

Same comment was added to this type of error, unify that and drop the
assertion as we'd find out quickly that something is wrong after
returning -EUCLEAN.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -1435,8 +1435,10 @@ static int find_parent_nodes(struct btrfs_backref_walk_ctx *ctx,
 	if (ret < 0)
 		goto out;
 	if (ret == 0) {
-		/* This shouldn't happen, indicates a bug or fs corruption. */
-		ASSERT(ret != 0);
+		/*
+		 * Key with offset -1 found, there would have to exist an extent
+		 * item with such offset, but this is out of the valid range.
+		 */
 		ret = -EUCLEAN;
 		goto out;
 	}