Fix buffer underflow if sysfs file is empty

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Jason Lee <jasonlee@lanl.gov>
Signed-off-by: Robert Evans <evansr@google.com>
Closes #16028
Closes #16035

cmd/zpool/os/linux/zpool_vdev_os.c

@@ -458,7 +458,7 @@ static char *zpool_sysfs_gets(char *path)
 	}
 
 	/* Remove trailing newline */
-	if (buf[count - 1] == '\n')
+	if (count > 0 && buf[count - 1] == '\n')
 		buf[count - 1] = 0;
 
 	close(fd);