Fix UB in SpiralIterator::operator++ (#481)

* Calling pop_back on an iterator past the end is UB

Signed-off-by: Ryan Friedman <[email protected]>
(cherry picked from commit 74dbbc8)

Author: Ryanf55

grid_map_core/src/iterators/SpiralIterator.cpp

@@ -66,6 +66,9 @@ const Eigen::Array2i & SpiralIterator::operator*() const
 
 SpiralIterator & SpiralIterator::operator++()
 {
+  if (isPastEnd()) {
+    return *this;
+  }
   pointsRing_.pop_back();
   if (pointsRing_.empty() && !isPastEnd()) {generateRing();}
   return *this;