Security fix: 2013 authentication exploit

Backported from MCPC+ 1.5.x import of fix from Spigot:

https://github.com/MinecraftPortCentral/MCPC-Plus-Legacy/commit/6348cd4e32a623db230c4b5834de9958ec490781#L1R111

For details see: https://github.com/MinecraftPortCentral/MCPC-Plus/issues/164

Important: MCPC+ for Minecraft 1.4.7 is no longer supported. For further support,
please update to the builds of MCPC+ for newer versions of Minecraft.
Merge pull request #1523 from MinecraftPortCentral/MC147-authfix

[1.4.7] Security fix: 2013 authentication exploit