resource: check for valid pointers

resource: check for valid pointers

This is patch is in continuation of the patchset
As mentioned by Madhan, this patch checks for the validity of
the pointers before their dereferences.

Change-Id: I9c1cd15bbcbd0a77bb34898ce3bc55c7d87b05f4
Signed-off-by: Ravi Nanjundappa <nravi.n@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2971
Reviewed-by: Sachin Agrawal <sachin.agrawal@intel.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Madan Lanka <lanka.madan@samsung.com>
Reviewed-by: Woochul Shim <woochul.shim@samsung.com>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>