Fix out-of-bounds access in ggwave_decode

[ggerganov]

ref #50

Also, provide a memory-safe overload called ggwave_ndecode()
The overload takes an extra parameter that specifies the size of
the output buffer and thus limits the size of the Rx payload that can be
decoded and stored.

[gizahNL]

Would it be possible to drop the nullbyte when using _ndecode? Our usecase (ideally) involves providing the address of a struct and a sizeof the struct, in our usecase _ndecode would never work.

[ggerganov]

Yes, actually that makes more sense now that I think about it.
Will change ggwave_ndecode to not add the null byte at the end.
Thanks for the suggestions!