Player interactions with the world can crash the server

[mathiascode]

After #3105, the server crashes when a player travels to another world. This is the output in console:

[18:55:36]   D:    | Cuberite has encountered an error and needs to close
[18:55:36] Details | SIGABRT: Server self-terminated due to an internal fault
[18:55:36] Cuberite Jenkins clang x64 Release (master) build id: #440
[18:55:36] from commit id: 0f2cebb7861516e1046a81c920a7e12be2747ac6 built at: Thu 31 Mar 11:45:14 CEST 2016
./Cuberite(_Z15PrintStackTracev+0x19)[0x5eb149]
./Cuberite[0x5d5115]
/lib/x86_64-linux-gnu/libc.so.6(+0x355a0)[0x7fec094245a0]
/lib/x86_64-linux-gnu/libc.so.6(gsignal+0x38)[0x7fec09424518]
/lib/x86_64-linux-gnu/libc.so.6(abort+0x16a)[0x7fec094260ea]
./Cuberite[0x5d511a]
/lib/x86_64-linux-gnu/libc.so.6(+0x355a0)[0x7fec094245a0]
./Cuberite(_ZN6cChunk12RemoveEntityEP7cEntity+0x2f)[0x5347af]
./Cuberite[0x80d2da]
./Cuberite(_ZN6cWorld15TickQueuedTasksEv+0x1b7)[0x5cb457]
./Cuberite(_ZN6cWorld4TickENSt6chrono8durationIlSt5ratioILl1ELl1000EEEES4_+0x289)[0x5b8029]
./Cuberite(_ZN6cWorld11cTickThread7ExecuteEv+0x6c)[0x5b7d0c]
/usr/lib/x86_64-linux-gnu/libstdc++.so.6(+0xb8c70)[0x7fec09a87c70]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x76fa)[0x7fec09d586fa]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7fec094f62dd]

[SafwatHalaby]

Sorry. I've converted some loops to c++11-style after testing my changes, so I didn't catch this. Currently the latest master is unstable and #3118 should be merged asap.

[SafwatHalaby]

Fixed. Are there any issues with the latest build?