panic with `SizeUnknown` used for creating a byte array

[adschwartz]

Running ebml-go I get this error:

panic: runtime error: makeslice: len out of range

This panic happens when the byte array is created here: https://github.com/at-wat/ebml-go/blob/master/value.go#L207

Looks the issue is related to this area of the code: https://github.com/at-wat/ebml-go/blob/master/value.go#L73

SizeUnknown is set to a large number 0xffffffffffffff that causes panic under certain conditions. I'm running this on mac os darwin which can potentially play a role. I wonder if a smaller number might be more reasonable.

	SizeUnknown = 0xffffffffffffff

https://github.com/at-wat/ebml-go/blob/ee4c93ced04df56f069a8bcbf0491659a403aa6a/value.go#L31