stack: Fixes cppcheck issue in ocserverrequest

stack: Fixes cppcheck issue in ocserverrequest

This patch fixes the the below issue reported by
cppcheck tool in stack dir:
"(error) Possible null pointer dereference:
ehResponse - otherwise it is redundant to check if
ehResponse is null at line 445
"

Dereferencing of ehResponse is done after NULL check.

Change-Id: I9cc20ce2386b5bdb08c4d9f9dbd2d143040f9a0f
Signed-off-by: Ravi Nanjundappa <nravi.n@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/1509
Reviewed-by: Erich Keane <erich.keane@intel.com>
Tested-by: Erich Keane <erich.keane@intel.com>