[IOT-2539] Clean sign-compare warning

[IOT-2539] Clean sign-compare warning

Use type casting to prevent the sign-compare warning
from occertutility.c code.

Bug: https://jira.iotivity.org/browse/IOT-2539
Change-Id: Ief40445dbb6445de246e9004aa42f4892a431d74
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21687
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>
(cherry picked from commit f3bd98df774da69d6d2b0b869bacea3f2c693e71)
Reviewed-on: https://gerrit.iotivity.org/gerrit/22209
Reviewed-by: Mats Wichmann <mats@linux.com>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>