Fix binary.Uvarint overflow.

[makhov]

@dvyukov found a bug with his go-fuzz

panic: runtime error: index out of range

goroutine 1 [running]:
github.com/golang/snappy/snappy.Decode(0x0, 0x0, 0x0, 0xc208041f00, 0xa, 0x20, 0x0, 0x0, 0x0, 0x0, ...)
    src/github.com/golang/snappy/snappy/decode.go:54 +0xaca
main.main()
    snappy.go:11 +0xf6

More info at #11.

[makhov]

There is no snappy project at https://go-review.googlesource.com/#/admin/projects/

[dvyukov]

@nigeltao Can you advise on this? Why is there no snappy project at go-review?

[nigeltao]

Thanks for the patch, but I don't think this is the right fix. Such input should return ErrCorrupt.

In any case, we should have a test as well.

I'll cook up a commit.