Fixed leak (Joerg: I'm not 100% sure on this one, can you check if what I did makes sense?)

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@6967 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/graphics/slip_stream.cpp

@@ -48,6 +48,8 @@ SlipStream::SlipStream(Kart* kart) : MovingTexture(0, 0), m_kart(kart)
 
     createMesh(m);
     m_node = irr_driver->addMesh(m_mesh);
+    m_mesh->drop();
+    
 #ifdef DEBUG
     std::string debug_name = m_kart->getIdent()+" (slip-stream)";
     m_node->setName(debug_name.c_str());