btrfs: check if there is space for chunk item when validating sys chunk array

We checked if have enough remaining space for a key before dereferencing a
key, but we then dereference a chunk item, to get the number of stripes,
without checking if there is space for the item. So add a check to see if
there is enough space for a chunk item before dereferencing the item to
extract the stripe count.

Fixes: 2a9bb78cfd36 ("btrfs: validate system chunk array at btrfs_validate_super()")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>