Don't drag multiple coincident vias

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17915

pcbnew/router/pns_dragger.cpp

@@ -196,6 +196,8 @@ const ITEM_SET DRAGGER::findViaFanoutByHandle ( NODE *aNode, const VIA_HANDLE& h
     if( !jt )
         return rv;
 
+    bool foundVia = false;
+
     for( ITEM* item : jt->LinkList() )
     {
         if( item->OfKind( ITEM::SEGMENT_T | ITEM::ARC_T ) )
@@ -211,7 +213,11 @@ const ITEM_SET DRAGGER::findViaFanoutByHandle ( NODE *aNode, const VIA_HANDLE& h
         }
         else if( item->OfKind( ITEM::VIA_T ) )
         {
-            rv.Add( item );
+            if( !foundVia )
+            {
+                rv.Add( item );
+                foundVia = true;
+            }
         }
     }
 

pcbnew/router/pns_index.h

@@ -142,6 +142,8 @@ int INDEX::Query( const ITEM* aItem, int aMinDistance, Visitor& aVisitor ) const
 {
     int total = 0;
 
+    wxCHECK( aItem->Kind() != ITEM::INVALID_T, 0 );
+
     const LAYER_RANGE& layers = aItem->Layers();
 
     for( int i = layers.Start(); i <= layers.End(); ++i )

pcbnew/router/pns_item.h

@@ -99,6 +99,7 @@ public:
     ///< Supported item types
     enum PnsKind
     {
+        INVALID_T   =    0,
         SOLID_T     =    1,
         LINE_T      =    2,
         JOINT_T     =    4,