btrfs-progs: util: Fix a wrong unit of pretty_size

btrfs-progs: util: Fix a wrong unit of pretty_size

If parameter for pretty_size is smaller than default base(1024),
pretty_size() will output wrong unit.
For example, pretty_size(1008) will output '0.98B' not '1008B' or
'0.98KiB'.

The cause is, for default base and auto-detect unit, base will be 1024
but num_divs is still 0, last result will still be divided by base,
causing the bug.

Fix it by checking num_divs in default case, and if num_divs is 0,
change base to 1.

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