TD: Ensure WalkerEdge is fully initialized

Coverity thinks that the edge_t is not getting fully initialized, so add a constructor to WalkerEdge that ensures the two components in question are set to zero (resulting in an invalid edge).

src/Mod/TechDraw/App/EdgeWalker.cpp

@@ -450,6 +450,13 @@ std::vector<edge_t> EdgeWalker::getEmbeddingRow(int v)
 //*******************************************
 // WalkerEdge Methods
 //*******************************************
+WalkerEdge::WalkerEdge()
+{
+    // Ensure the edge is properly initialized (Coverity defect 316559)
+    ed.m_source = 0;
+    ed.m_target = 0;
+}
+
 bool WalkerEdge::isEqual(WalkerEdge w)
 {
     if ((v1 == w.v1 && v2 == w.v2)  ||

src/Mod/TechDraw/App/EdgeWalker.h

@@ -71,14 +71,15 @@ using planar_embedding_t =
 class TechDrawExport WalkerEdge
 {
 public:
+    WalkerEdge();
     static bool weCompare(WalkerEdge i, WalkerEdge j);
     bool isEqual(WalkerEdge w);
     std::string dump();
 
-    std::size_t v1;
-    std::size_t v2;
+    std::size_t v1 {0};
+    std::size_t v2 {0};
     edge_t ed;
-    std::size_t idx;
+    std::size_t idx {0};
 };
 
 class TechDrawExport ewWire