Fixed missing null terminator in mpm_sample_client

Fixed missing null terminator in mpm_sample_client

The string in contained in the MPMMessage passed to the onCallback
funtion may not be null terminated.

This is accounted for when creating the message_char variable but
the message_char variable was not used when MPMAddDevice was called
causing a non-terminated string to be passed to the to the plugin.

Using the message_char variable with size+1 solved the missing null
terminated string issue.

Change-Id: If760cc77fba9fe46b191b3c5a7ec2abac9da63ac
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18491
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Todd Malsbary <todd.malsbary@intel.com>