Update decode.go

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From https://sites.google.com/site/musicgapi/technical-documents/wav-file-format.

One tricky thing about RIFF file chunks is that they must be word aligned. This means that their total size must be a multiple of 2 bytes (ie. 2, 4, 6, 8, and so on). If a chunk contains an odd number of data bytes, causing it not to be word aligned, an extra padding byte with a value of zero must follow the last data byte. This extra padding byte is not counted in the chunk size, therefor a program must always word align a chunk headers size value in order to calculate the offset of the following chunk.