Function paths with invalid characters results in game freezing

The bug

When loading a world with a function containing a file name containing an invalid character, the game freezes and produces the following error in the output log

MinecraftServer    Encountered an unexpected exception, g: Non [a-z0-9/._-] character in path of location

Example path:

custom/foo [error here]/bar.mcfunction

This causes the game to freeze as `[` is an invalid character.