Do not iterate over the map when size is 0

[Smiffe]

Hey,

I have noticed small bug within GridMapIterator. There are two situations in which GridMap object is empty (size is 0,0):

• When geometry of GridMap is not set, so it's by default (0,0)
• Due to very small length values, so size may be rounded to (0,0) https://github.com/ANYbotics/grid_map/blob/master/grid_map_core/src/GridMap.cpp#L51

When using GridMapIterator as shown in README.md and demos https://github.com/ANYbotics/grid_map/blob/master/grid_map_demos/src/IteratorsDemo.cpp#L58, first iteration is always valid as isPastEnd_ variable is false regardless of the GridMap's size https://github.com/ANYbotics/grid_map/blob/master/grid_map_core/src/iterators/GridMapIterator.cpp#L20. Accessing empty GridMap leads to exception which crashes whole application as currently it's not catched or even documented.

I have added validation in GridMapIterator constructor which ensures GridMap is accessible. I have also added simple test to check if isPastEnd_ variable initializes with true value when passed GridMap is empty.

[maximilianwulf]

Thank you, will import it.