btrfs: handle extent corruption with select_one_root properly

In corruption cases we could have paths from a block up to no root at
all, and thus we'll BUG_ON(!root) in select_one_root.  Handle this by
adding an ASSERT() for developers, and returning an error for normal
users.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>