The set_edges() setter was broken as was missing a negation for the
if-statement protecting the access of the edges member. Fix it by
adding the missing negation.

There existed no get_next_edge() function to access the next_edge
member in the edge struct, so add it to make it to provide an
abstraced way of accessing it.

Signed-off-by: Simon Sundberg <simon.sundberg@kau.se>