Add testcase that shows the problem

Segment_Delaunay_graph_2/test/Segment_Delaunay_graph_2/issue7972.cpp

@@ -0,0 +1,21 @@
+
+#include <CGAL/Segment_Delaunay_graph_2.h>
+#include <CGAL/Segment_Delaunay_graph_traits_2.h>
+#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
+
+typedef CGAL::Exact_predicates_exact_constructions_kernel K;
+typedef CGAL::Segment_Delaunay_graph_traits_2<K>          Gt;
+typedef CGAL::Segment_Delaunay_graph_2<Gt>                SDG2;
+
+int main() {
+    auto segments = std::vector({
+        CGAL::Segment_2<K>(
+            CGAL::Point_2<K>(0.0, 0.0),
+            CGAL::Point_2<K>(1.0, 0.0))
+    });
+
+    SDG2 delaunay;
+    delaunay.insert_segments(segments.begin(), segments.end());
+
+    return 0;
+}