Fixed memory leak.

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@12019 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/graphics/irr_driver.cpp

@@ -783,6 +783,7 @@ scene::ISceneNode* IrrDriver::addWaterNode(scene::IMesh *mesh,
                                                      wave_height, wave_speed,
                                                      wave_length);
     out->getMaterial(0).setFlag(EMF_GOURAUD_SHADING, true);
+    welded_mesh->drop();  // The scene node keeps a reference
     return out;
 }   // addWaterNode