fix: anticipate non-normal edges when constructing ortho splines

When splines=ortho and concentrate=true were used in combination, scenarios could occur when spline construction would dereference a null pointer. The root cause was the logic touched in this change, that attempted to reverse an edge back to its originating copy, while assuming that the originator was NORMAL.

As implied by 2c07d238 and 84e468e7 on which this change is based, there are almost certainly further remaining problematic ED_edge_type loops in the code base.

Gitlab: fixes #2361 (closed)
Reported-by: forum user jmetz