modified if condition for log in messagehandler.c

modified if condition for log in messagehandler.c

null pointer leads to undefined behavior.
so if condition has to be modified.

Change-Id: Idf906ec940a80547543dfe01ccc9cbcda0f1d9b8
Signed-off-by: hyuna0213.jo <hyuna0213.jo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2178
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>
Reviewed-by: Erich Keane <erich.keane@intel.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Naga Ashok Jampani <jn.ashok@samsung.com>