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