Fix possible future overflow in zfs_nicenum

The function zfs_nicenum that converts number to human-readable output
uses a index to a string of letters. This patch limits the index to
the length of the string.

Signed-off-by: Christer Ekholm <che@chrekh.se>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3122

lib/libzfs/libzfs_util.c

@@ -571,7 +571,7 @@ zfs_nicenum(uint64_t num, char *buf, size_t buflen)
 	int index = 0;
 	char u;
 
-	while (n >= 1024) {
+	while (n >= 1024 && index < 6) {
 		n /= 1024;
 		index++;
 	}