Bugfix for parsing std_msgs/Empty definition
Bugfix for parsing std_msgs/Empty definition

The check for the message type on the first line of the definition was
failing; when the message definition is empty, it has 0 lines.  Add a
size check when parsing the fields.

Fixes #10

bcharrow/matlab_rosbag,zheng-rong/matlab_rosbag,bcharrow/matlab_rosbag,zheng-rong/matlab_rosbag,bcharrow/matlab_rosbag,zheng-rong/matlab_rosbag