FIX: tree support crashes when there is no overhang

Default infill pattern is lightning now, which has
a crash bug when no overhang is provided.

Change-Id: I1e70b8458d1eef670a78d698c68b21d7f48bc3eb
(cherry picked from commit 777af3aa1dbb9869209945f3a6dbcc07cba0b84a)

src/libslic3r/Fill/Lightning/Generator.cpp

@@ -213,6 +213,8 @@ void Generator::generateTrees(const PrintObject &print_object)
 
 void Generator::generateTreesforSupport(std::vector<Polygons>& contours)
 {
+    if (contours.empty()) return;
+
     m_lightning_layers.resize(contours.size());
     bboxs.resize(contours.size());