Validate the buffer_ of CustomServiceRequest object before using it to (#210)
Validate the buffer_ of CustomServiceRequest object before using it to (#210)

instantiate class Cdr

Before we are going to instantiate the fastcdr::Cdr class by using the member
variable buffer_ of class CustomServiceRequest, we must ensure that the buffer_
can not be nullptr.

This patch implements the validation.

Fix #209
eProsima/ROS-RMW-Fast-RTPS-cpp,ros2/rmw_fastrtps,ros2/rmw_fastrtps