Fixed a crash when moving a player to a non-existent world (reported by tonibm19)

http://forum.mc-server.org/showthread.php?tid=434&pid=8865#pid8865 crashdump.zip/crash_mcs_165c.dmp

git-svn-id: http://mc-server.googlecode.com/svn/trunk@1645 0a769ca7-a7f5-676a-18bf-c427514a06d6