pcbnew: Only regenerate BBox if the item is valid

Items removed from the connectivity list will not have valid parents, so
we cannot update the BBox, thus we will only use the cached value.

Fixes: lp:1785605
* https://bugs.launchpad.net/kicad/+bug/1785605

pcbnew/connectivity_algo.h

@@ -338,7 +338,7 @@ public:
 
     const BOX2I BBox()
     {
-        if( m_dirty )
+        if( m_dirty && m_valid )
         {
             EDA_RECT box = m_parent->GetBoundingBox();
             m_bbox = BOX2I( box.GetPosition(), box.GetSize() );