Trying to acess a non-existing frontier

[jmfrancolin-gatech]

Often the planner tries to access non-existing frontiers on the array of available frontiers. As a consequence, the code crashes with the standard python message "list index out of bounds".

According to our battery of tests, this issue happens often and in otherwise ideal conditions in which we should not observe such a problem. More specifically, the code was tested in a physical turtlebot attempting to map a square space with walls always within the range of the kinetic sensor and one opening (the theoretical goal of the robot).