Don't continue with a null board in board import

Some plugin is failing and returning a null-board when the usual convention was exceptions

Fixes KICAD-3ZJ

pcbnew/files.cpp

@@ -678,7 +678,7 @@ bool PCB_EDIT_FRAME::OpenProjectFiles( const std::vector<wxString>& aFileSet, in
             failedLoad = true;
         }
 
-        if( failedLoad )
+        if( failedLoad || !loadedBoard )
         {
             // We didn't create a new blank board above, so do that now
             Clear_Pcb( false );