Fix #3955 (player dying on login).

It was caused by player not moving because fall was prevented, but their
velocity still increasing, causing fatal fall damage when world was
finally loaded. This commit fixes it by setting player velocity to zero
when the world around them is not loaded.

src/collision.cpp

@@ -332,8 +332,10 @@ collisionMoveResult collisionMoveSimple(Environment *env, IGameDef *gamedef,
 
 	// Do not move if world has not loaded yet, since custom node boxes
 	// are not available for collision detection.
-	if (!any_position_valid)
+	if (!any_position_valid) {
+		*speed_f = v3f(0, 0, 0);
 		return result;
+	}
 
 	} // tt2