Fix "Add colored output to zfs list"

Running zfs list -o avail rpool resulted in a core dump.
This commit will fix this.

Run the needed overhead only, when use_color() is true.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #14712