Fix NPE when loading second single-player world

After leaving a single-player world, getServer() can return null, resulting
in the stack trace below, "Fatal errors were detected during the
transition from SERVER_STOPPING to SERVER_ABOUT_TO_START",
"The ForgeModLoader state engine has become corrupted" and the client
getting stuck forever.

Exception in thread "Server thread" java.lang.NullPointerException
at cpw.mods.fml.client.FMLClientHandler.serverStopped(FMLClientHandler.java:620)
at cpw.mods.fml.common.FMLCommonHandler.handleServerStopped(FMLCommonHandler.java:468)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:583)
Merge pull request #284 from smcv/uninit

Fix NPE when loading second single-player world