Reset Tux' velocity after using a door (SuperTux#3279)

This avoids taking any movement from last sector to the new one.

Fixes SuperTux#3272

Author: Brockengespenst

src/supertux/game_session.cpp

@@ -603,6 +603,9 @@ GameSession::update(float dt_sec, const Controller& controller)
 
       if (m_spawn_with_invincibility)
       {
+        // Reset velocity to avoid taking any movement from last sector to new one
+        p->set_velocity(0.0f, 0.0f);
+
         // Make all players temporarily safe after spawning
         p->make_temporarily_safe(SAFE_TIME);
       }