A possible fix to crash in

#0  0x0296dab6 in std::_Rb_tree_increment ()
#1  0x002762b3 in std::_Rb_tree_const_iterator<Cell*>::operator++ () at stl_tree.h:351
#2  0x002762b3 in CellComplex::reduction (this=0x1a854d80, dim=2) at CellComplex.cpp:351