Initialize all fields of zero token before returning it.

@pushrax for review

## Problem

I was getting the following compilation error when trying to compile the gem on Mac OS X (I guess llvm complains about this but not gcc

```
parser.c: In function ‘parser_consume’:
parser.c:39: warning: ‘zero.val_end’ is used uninitialized in this function
parser.c:39: warning: ‘zero.val’ is used uninitialized in this function
parser.c:39: warning: ‘zero.flags’ is used uninitialized in this function
make: *** [parser.o] Error 1
```

This isn't currently a bug, since we were only using the `type` field in the only call site for `parser_consume`

## Solution

Zero initializing all the fields in the struct avoids this compilation issue.