math: derive marshalled byte length from copy, not blind assumptions

[odeke-em]

The specification of "copy", the builtin function per
https://pkg.go.dev/builtin#copy, says that it returns the minimum of
len(src) and len(dst) when invoked as:

 copy(dst, src)

of which the prior code blindly assumed that everytime that
copy is invoked that the buffer provided had enough size
to accomodate the contents of *.MarshalTo but this isn't true
at all if len(data) is less than the values of .Marshal()

[julienrbrt]

Thanks! Good find!
Any reason why we don't just return as it's using named return parameters?