PcbNew; fix crash on copy/paste

Due to a bug introduced by commit ba1e0efda that tried to fix an other bug.

Fixes: lp:1771003
https://bugs.launchpad.net/kicad/+bug/1771003

pcbnew/tools/pcbnew_control.cpp

@@ -942,6 +942,9 @@ static void moveNoFlagToVector( DLIST<T>& aList, std::vector<BOARD_ITEM*>& aTarg
 
 static void moveNoFlagToVector(  ZONE_CONTAINERS& aList, std::vector<BOARD_ITEM*>& aTarget, bool aIsNew )
 {
+    if( aList.size() == 0 )
+        return;
+
     auto obj = aList.front();
     int idx = 0;