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()
This change adds a negative value check to Uint.Unmarshal,
which coincidentally is fixed by refactoring for code reuse.
While here, added tests to ensure we don't regress.
Fixes#11995
