btrfs-progs: Check the csum tree node before go through the csum tree

btrfs-progs: Check the csum tree node before go through the csum tree

[BUG]
Some fsfuzzed btrfs image will cause btrfsck segfault.

[REPRODUCER]
Run btrfsck on a csum tree block corrupted image.

[REASON]
check_csums() function call btrfs_search_slot() on csum_tree but doesn't
check whether the csum_tree contains a valid extent_buffer, which causes
the segfault.

[FIX]
Check the csum_root->node before any search.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>