pcbnew: Check for valid pointer before deref

In legacy toolkit, we do not keep the corner pointers, so dereferencing
causes crashes.

Fixes: lp:1808852
* https://bugs.launchpad.net/kicad/+bug/1808852

pcbnew/class_zone.h

@@ -207,7 +207,9 @@ public:
     int GetSelectedCorner() const
     {
         // Transform relative indices to global index
-        int globalIndex;
+        int globalIndex = -1;
+
+        if( m_CornerSelection )
             m_Poly->GetGlobalIndex( *m_CornerSelection, globalIndex );
 
         return globalIndex;