Display random objects independently of buildings

Fix bug reported by Thorsten RENK on the mailing list
that random objects were only being displayed if random
objects were also present for the tile.  Also fix crash
if none of the random object models were found.

simgear/scene/material/matmodel.cxx

@@ -130,6 +130,8 @@ SGMatModel::load_models( SGPropertyNode *prop_root )
         
       } else {
         SG_LOG(SG_INPUT, SG_ALERT, "Failed to load object " << _paths[i]);
+        // Ensure the vector contains something, otherwise get_random_model below fails
+        _models.push_back(new osg::Node());
       }
     }
   }

simgear/scene/tgdb/SGTileDetailsCallback.hxx

@@ -387,7 +387,7 @@ public:
             float cos_zero_density_angle = mat->get_cos_object_zero_density_slope_angle();
             float cos_max_density_angle  = mat->get_cos_object_max_density_slope_angle();
             
-            if (building_coverage == 0)
+            if ((building_coverage == 0) && (group_count ==0))
                 continue;
             
             SGBuildingBin* bin = NULL;