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/TreeSupport.hpp

@@ -351,7 +351,7 @@ public:
 
     int  avg_node_per_layer = 0;
     float nodes_angle       = 0;
-    bool            has_sharp_tail;
+    bool  has_overhangs = false;
 
     std::unique_ptr<FillLightning::Generator> generator;
     std::unordered_map<double, size_t> printZ_to_lightninglayer;