Werror=maybe-uninitialized in vmod_decode()

In vmod_decode() h is declared but then is used without being initialized at
https://github.com/fastly/libvmod-urlcode/blob/master/src/vmod_urlcode.c#L111

When built with -Wall -Werror, this leads to:

```
vmod_urlcode.c: In function 'vmod_decode':
vmod_urlcode.c:74:6: error: 'h' may be used uninitialized in this function [-Werror=maybe-uninitialized]
```
