std::map loop using for is wrong in GetMeanIntensity() when dealing wth labelimage which contains non contiguous labels. Mantis #ref564

BUG: Drivername may be valid, but OGR still can not create the driver because it has been compiled withtout the appropriate dependency. Avoiding segmentation fault in this case

COMP: Fixing warning 'i' : definition from the for loop is ignored; the definition from the enclosing scope is used