btrfs: fix error pointer check in __btrfs_map_block()

The btrfs_get_chunk_map() never returns NULL, it returns error pointers.

Fixes: 89b798a ("btrfs: Use btrfs_get_io_geometry appropriately")
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6023,7 +6023,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
 		return ret;
 
 	em = btrfs_get_chunk_map(fs_info, logical, *length);
-	ASSERT(em);
+	ASSERT(!IS_ERR(em));
 	map = em->map_lookup;
 
 	*length = geom.len;