btrfs: add assertions where number of devices could go below 0

In preparation to add helper function to deduce the num_devices with
replace running, use assert instead of BUG_ON or WARN_ON. The number of
devices would not normally drop to 0 due to other checks so the assert
is sufficient.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog, adjust the assert condition ]
Signed-off-by: David Sterba <dsterba@suse.com>