Invalid group_centers in Flood Fill

[jara001]

In case that something bad happens in the cascade, it may happen that points that are outside of the created map are used as group_centers.

There should be a check for this, as

_map[tuple(_c)] = _i

is not enough.

However, what to do in this case is still left to decide.

Note: This should not be present in euclidean segmentator, as it works differently.