pcbnew: Fix VRML export checking

In rare cases, the VRML triangulator may claim to work but return empty
vectors.  These cases need to be caught before we dereference the vector
elements.

Fixes: lp:1838448
* https://bugs.launchpad.net/kicad/+bug/1838448

pcbnew/exporters/export_vrml.cpp

@@ -1702,7 +1702,8 @@ static void create_vrml_plane( IFSG_TRANSFORM& PcbOutput, VRML_COLOR_INDEX color
     std::vector< int > idxPlane;
     std::vector< int > idxSide;
 
-    if( !(*layer).Get2DTriangles( vertices, idxPlane, top_z, aTopPlane ) )
+    if( !( *layer ).Get2DTriangles( vertices, idxPlane, top_z, aTopPlane ) || idxPlane.empty()
+            || idxSide.empty() )
     {
 #ifdef DEBUG
         do {
@@ -1788,7 +1789,8 @@ static void create_vrml_shell( IFSG_TRANSFORM& PcbOutput, VRML_COLOR_INDEX color
         bottom_z = tmp;
     }
 
-    if( !(*layer).Get3DTriangles( vertices, idxPlane, idxSide, top_z, bottom_z ) )
+    if( !( *layer ).Get3DTriangles( vertices, idxPlane, idxSide, top_z, bottom_z )
+            || idxPlane.empty() || idxSide.empty() )
     {
 #ifdef DEBUG
         do {