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

lib/libzfs/libzfs_util.c

@@ -1965,7 +1965,7 @@ zfs_version_print(void)
  * Return 1 if the user requested ANSI color output, and our terminal supports
  * it.  Return 0 for no color.
  */
-static int
+int
 use_color(void)
 {
 	static int use_color = -1;