Btrfs-progs: check return value of read_tree_block() in check_chunks_and_extents()

Btrfs-progs: check return value of read_tree_block() in check_chunks_and_extents()

The following steps could trigger btrfs segfault:

mkfs -t btrfs -m raid5 -d raid5 /dev/loop{0..3}
losetup -d /dev/loop2
btrfs check /dev/loop0

The reason is that read_tree_block() returns NULL and
add_root_to_pending() dereferences it without checking it first.

Also replace a BUG_ON with proper error checking.

Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>