Fix build warnings on ocstack.c
Fix build warnings on ocstack.c

Bug: https://jira.iotivity.org/browse/IOT-2270
Change-Id: I19c88fd57bcaa8931921f0e23dd55547c84818e3
Signed-off-by: Ziran Sun <3baa2a3e3c6d96dba4b6f1736d53815847db9a07@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/20031
Tested-by: jenkins-iotivity <d95b56ce41a2e1ac4cecdd398defd7414407cc08@iotivity.org>
Reviewed-by: Phil Coval <ff46fec9739d31cce915aabcc3a7a2c00eee4d6c@osg.samsung.com>
Reviewed-by: Jaewook Jung <e30c770b7f9b30402e4d6c2240cd5b03cb3bf7f9@samsung.com>

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