MoonScript 0.3.0 parser enters an inconsistent state after an unsuccessful chunk parsing

[shmuz]

Running the following code

to_lua = require("moonscript.base").to_lua
to_lua "{b=5}"
to_lua "{b=5}"

produces the following error:

moon: [string "moonscript.lua"]:2854: attempt to call upvalue 'pos' (a number value)
stack traceback:
[string "moonscript.lua"]:2854: in function 'build_grammar'
[string "moonscript.lua"]:2975: in function 'string'
[string "moonscript.lua"]:390: in function 'to_lua'
2.moon:3: (3) in main chunk

while I was expecting just returning nil.

[szensk]

@leafo line 334 in parse.moon should be local pos = last_pos if I understand correctly

[leafo]

Thanks, will push out 0.3.1 in a bit.