Unable to define frontiers when there are no features within the range of the Kinect sensor

[jmfrancolin-gatech]

The maximum range of the Kinect sensor is about 3.5 meters. If the turtlebot is placed on a scenario in which there's no obstacles within that range, the code is unable to select a new frontier and therefore it crashes.

This is particularly problematic when the turtlebot is mapping an environment such as an office and it finds itself in a corridor, for example, where it cannot find any features within its max range, and therefore it crashes.

Another obvious implication is the fact that under the current code-base the turtlebot is unable to map open spaces.