Allow GetLayer() to be used with single-layer zones.

Also fixes a bug in GetFirstLayer() where we were checking
m_layerSet.size() (which is always 60) rather than
m_layerSet.count() (which is the number of set layers).

Fixes https://gitlab.com/kicad/code/kicad/-/issues/15233

common/layer_id.cpp

@@ -32,6 +32,9 @@ wxString LayerName( int aLayer )
     switch( aLayer )
     {
     // PCB_LAYER_ID
+    case UNDEFINED_LAYER:   return _( "undefined" );
+
+    // Copper
     case F_Cu:              return wxT( "F.Cu" );
     case In1_Cu:            return wxT( "In1.Cu" );
     case In2_Cu:            return wxT( "In2.Cu" );

pcbnew/zone.cpp

@@ -247,13 +247,16 @@ VECTOR2I ZONE::GetPosition() const
 
 PCB_LAYER_ID ZONE::GetLayer() const
 {
-    return BOARD_ITEM::GetLayer();
+    if( m_layerSet.count() == 1 )
+        return m_layerSet.UIOrder()[0];
+    else
+        return UNDEFINED_LAYER;
 }
 
 
 PCB_LAYER_ID ZONE::GetFirstLayer() const
 {
-    if( m_layerSet.size() )
+    if( m_layerSet.count() )
         return m_layerSet.UIOrder()[0];
     else
         return UNDEFINED_LAYER;