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

include/libzutil.h

@@ -163,6 +163,7 @@ int zfs_ioctl_fd(int fd, unsigned long request, struct zfs_cmd *zc);
 #define	ANSI_RESET	"\033[0m"
 #define	ANSI_BOLD	"\033[1m"
 
+int use_color(void);
 void color_start(const char *color);
 void color_end(void);
 int printf_color(const char *color, char *format, ...);